My oldest sister, Abby, calls for the second time this morning. “Oh, Jannie!” she says, reverting to my baby-sister nickname. “It’s so great to hear your voice, finally! Are you in Idaho now?”

“We sure are,” I answer, in the same tone that I answered the same question earlier. It’s been over a year since I relocated, not long after I helped Abby and her husband transition to assisted living. I can picture her perched on the loveseat in their tiny apartment, its walls vivid with her paintings, sun streaming in from windows that look out over California foothills. “How are you guys?” I ask.

“Oh, fine! We just got back from … where we eat.” She means the dining room, but she falters. Then, brightening: “Are you in Idaho now?”

Our conversations circle like this now, their loops gradually shortening, spiralling cheerfully along the surface.

According to Alzheimer’s Association, Abby is one of 6.5 million Americans living with the disease. When she was diagnosed, I was heartsick. One of the few consolations I clung to was that my other sister, Sal, and I could sustain each other through the long goodbye.

But it turns out that Alzheimer’s is not the only type of dementia. Others include dementia with Lewy bodies, vascular dementia and Huntington’s disease. There are other conditions that cause mental confusion or disorientation, sometimes temporarily, as can occur in older people who have urinary tract infections. With other afflictions, dementia is permanent and progressive. One such disorder is corticobasal degeneration. I discovered that when Sal, my next-oldest sister, was diagnosed.

Growing up, I was the baby of the family. Abby, 12 years my senior, was for me the embodiment of glamour. Sal, seven years older than me, was my mentor and de facto nanny. I dogged both my big sisters’ steps as much as they’d let me, observing them with a mixture of envy and hero worship. I revelled in their attention when they would dress me up like a life-size doll, until I grew restless and they’d hiss at me to “hold still!” Other times, when they thought I was getting off too easily for whatever annoyance I’d caused, they’d take me out of Mom’s earshot and scold me themselves. It worked: my sisters were my role models, and I craved their approval more than I did that of my parents or, later, even my friends.

I thought Abby could do anything. Sidestepping our domineering father, she put herself through college, becoming an artist and teacher. I was five when she left home, and our house felt semi-becalmed in her absence.

After Abby left, Sal did nearly as much to raise me as did my mother, who so often deflated in the face of my father’s vehemence. It was Sal who taught me how to clean a bathroom, how to bake a cake, how to be a good friend, and how to drive her ’67 Camaro the summer I turned 16 and she was home for a visit. Later, as the three of us were figuring out how to raise our sons — coming from a family of all girls, we were somewhat mystified as we produced only boys — it was Sal’s house where we gathered for holidays and celebrations, for the memorials when our parents died.

Abby was in her mid-60s when her formidable mind showed signs of slipping. It took years, several fraught family meetings, and a long series of neurological assessments before Sal and I, along with the rest of Abby’s loved ones, had to face the reality of her condition. Like many people, we’d been schooled to believe that Alzheimer’s was something to be dreaded, a fate possibly worse than death.

As devastated as we both were about Abby, Sal was confronted with another life cataclysm when her 40-year marriage blew up. It turned out her steady, taciturn husband had been a serial philanderer for a dozen years. She emerged shaken and raw, as though whatever emotional insulation she’d had was wearing away, leaving her newly fragile.

Then one day I got a frantic call from the closest in Sal’s large circle of friends. “Something’s wrong,” the friend said. “Come see for yourself.”

I still lived in California at the time, 300 miles up the coast from where Sal lived. I drove down the next day. Sal greeted me at her front door, delighted to see me but startlingly thin. Her posture was hunched, her left arm contracted at an odd angle. A bruise, unsuccessfully disguised with makeup, spread across one side of her face.

“I had a little fall down the stairs,” she explained with a nervous laugh. “It’s not a big deal, nothing’s broken.” Her affect as well as her body seemed so frail that I kept my alarm to myself, but after our visit I conferred with her oldest son, who lived nearby. He too was concerned. He’d recently taken her out to her favourite steak place, where she’d had no idea that she’d rested her left arm in the middle of her entree until he pointed it out to her.

It took months for Sal to get appointments with a battery of neurologists, and even longer for the doctors to confer. Eventually, they agreed on a diagnosis. All the signs pointed to corticobasal degeneration, sometimes referred to as corticobasal syndrome.

I’d never heard of either. “Is that serious?” I asked Sal when she told me.

“It’s not good,” she answered. “But I’m going to make the best of it.” Her voice trembled with mixed fear and resolve, already weaker than it had been a few months earlier. “I’ll be OK,” she said. My heart cracked, understanding for the first time that she wouldn’t be.

I didn’t want to plague Sal with questions, so I pored over what little information I could find. What I learned is that the best to be said of corticobasal degeneration is that it is rare. It blights the brain’s cortex and basal ganglia, causing loss of balance and muscle control, impaired speech, the eerie “alien limb syndrome” — which explained how Sal’s arm wound up in her prime rib — and, as the literature blandly stated, “changes in thinking and personality.” It is progressive, incurable, and, unlike Alzheimer’s or Parkinson’s, no medication exists that slows it down. Ultimately, it is fatal. I wanted to creep into a cave, a refuge where I could process this new upheaval.

But Abby, once she’d learned that Sal was ailing, had many questions — rather, the same questions over and over, with which she besieged Sal over the phone. As the suddenly most capable sister, it fell to me to deflect her well-meaning assaults.

“What’s the name of what she’s got? Let me write it down,” Abby would ask every time we talked. Her refrigerator fluttered with Post-its, all of them with the name of Sal’s malady as I’d dictated it, inscribed in Abby’s graceful script.

Abby was as determined to fix Sal as she was unable. She obsessed over her scheme to move in with her now-disabled sister, leaving her husband — never able to countermand Abby’s will — at home. Sal was aghast but had neither the energy nor the temperament to head Abby off.

Again, intervention fell to me. It demanded new skills, including therapeutic lying, as I invented reasons why the upcoming week wouldn’t be a good time for Abby to go stay with Sal, week after week, month after month.

These days, so long as I don’t bring up Sal and her illness when we visit, Abby seems happy. Her body is strong, she can draw and paint, and thanks to her devoted husband, she doesn’t require the kind of memory care that is designed to both protect and restrict patients who are prone to wandering. She loves her new home, including the dining room where, as far as she’s concerned, the menu changes with every meal. It’s possible that Abby is more content now than she was when her brain was firing on all its cylinders.

Sal’s decline, by comparison, is cruel. I’ve helped her son move her to upgraded care three times as she has diminished. Her current facility is the best available in her area, providing round-the-clock care. She can summon help with the push of a button on a lanyard around her neck. But she has so little control over her limbs now that sometimes she can’t find the button, or she pushes the TV remote instead. She can no longer walk, stand or sit up unaided. She insists on feeding herself but it’s difficult to watch; choking is always a danger. Her speech has further weakened, her words slurred and halting to the point where they’re often indecipherable.

I travel to visit her, and within minutes of my arrival she makes plaintive requests, as poignant for their simplicity as they are unrealistic.

“Can you bring me my sewing machine? What if we hop in your car and go to lunch?” she asks, her blue eyes wide with hope. The disease’s assault on her brain leaves her unable to register the scope of her disability or relinquish her identity as someone who can operate a sewing machine or hop in a car. It’s a symptom that causes more distress — for her, for her caregivers, for her loved ones — than her physical limitations.

I manage to change the subject every time, and we laugh together over the stories she recounts from our childhood. But soon she tires, her eyes unfocused and half-shut, her words devolving into incoherent mumbling. Still, she continues talking on and on. Ashamed of my limited patience, I find an excuse to leave, assuring her I’ll return soon.

I don’t talk much about this. Chronic, long-lasting tragedy is unnerving; nobody knows what to say in response to my stories. Friends tell me I’m strong, that I’m handling it well. I am not, I want to say but don’t: I’m a lost little sister wandering the shores of calamity.

But I’m no longer the baby of the family, so I must do what I can. For Abby, I can meet her where she is, revisiting each topic or question however she reframes and repeats it, again and again. For Sal, I can hang on a little longer when I can’t understand a word she says, or when her descent is so dizzying it makes me want to escape rather than bear witness to it.

In my weaker moments, I fret over an unanswerable question: will my brain fail me too? I don’t often allow myself to go there. My sisters’ dementias have flipped our birth order, and it’s now my place to be the steady one, the big sister among us, for whatever time we have left together.

Jan M. Flynn’s essays appear on Medium.com and on her blog at JanMFlynn.net. Her short fiction has won international awards and appears in literary journals including Midnight Circus, The Binnacle, Noyo River Review, Far Side Review, Grim and Gilded, and Bullshit Lit as well as anthologies. She is also the host and producer of a weekly podcast, “Here’s A Thought,” for people who overthink. She lives in Boise, Idaho, and is represented by Helen Adams of Zimmermann Literary, New York.