Never seen that actually happen. More likely is a diffusion of responsibility and one sibling drawing the short stick and doing all the work.

It is not even conscious malice by the other siblings. Stuff like that happens naturally. All siblings would need to live close by, have similar family circumstances (I have a family on my own that I need to prioritize. Betty who is single can take care of dad.) and similar level of skills in caring for the parent for that to work.

Also people underestimate how horrible caring for people with dementia and other disabilities that require a high level of care is for people not trained in it. It is not socially acceptable to talk about the bad stuff.

This (along with odds of survival) was traditionally the main reason for having many children. They were help around the farm but also a form of life insurance.

Also as in the case of my mother’s family (11 kids), the onus for taking care of the younger siblings progressively falls on the siblings older than them.

Spread?!

My wife is the first of seven. Yes, she was the one financially most able to help her parents, both from being here rather than there and because she has a strong work ethic, unlike her siblings. That doesn't mean the load should have fallen virtually 100% on her when the other 6 were in the same city, not on the other side of the world.

Even in large families I know of, the burden usually rests on one or two - but the larger the family the more support of the burden-bearers there are, which can make a huge difference.