Research shows that even when there are several adult children, just one (a daughter, usually) emerges as the so-called primary caregiver and handles the bulk of her parents’ care. But Ms. Milgram-Bossong finds that minor consolation.

“Even if I had a sibling in California, I’d have someone to talk to. I have no one to bounce ideas off, to think about what I’m not thinking about,” she said. Widowed in her 50s with no children, she’s determined to do well by her parents when they need her, but the prospect keeps her awake nights.

“My brother has a lot invested in denial: ‘They’re fine, they’re fine, they’re fine.’ My sister wants to keep them in that house, so she can move there when she retires.” When Ms. Reiss explains to her sister that the house will most likely be sold to finance their parents’ care, “she scowls and storms off.” She added, “A lot of hostility can flare up over this.” Her parents made Ms. Reiss their executor and trustee, but as the youngest sibling, she’s finding that her brother and sister resent her authority.

Glenna Mills, a San Francisco sculptor, has seen this firsthand. Disputes about who would take charge of daily decision-making for her parents, in assisted living in Idaho, grew so explosive that now three of seven siblings no longer visit or talk to their parents at all — or the brothers and sisters they disagree with. “It’s horrifying,” Ms. Mills told me in an interview. “We were always an extremely close family.”

Read more in the New York Times.