This is something that seems to happen a lot in families. My oldest sister passed away over 15 years ago, so the responsibility falls to me, the youngest.

With friends of mine, a pair of siblings, the older one who is single with no kids had the caregiver role for their now deceased mom, while the younger one who was married with school aged kids bowed out. With one of their sets of cousins it was the youngest child, a later in life kid in the family, who had to take his parents into his house and he has a wife and younger school aged kids. His mom has Alzheimer's and dad has diabetes and other health problems. His older siblings have kids who are older- high school, college or beyond, but they have zero interest in helping out.

Unfortunately there is little in the way of resources out there when family members refuse to help which is why people who are full time caregivers tend to wind up in worse health versus the general population, and some even predecease the relatives they care for.