Is your niece a Christian? Would she be open to pastoral counseling? My advice would be to pray for the situation and give the niece space.

As someone dealing with cancer - I also often make decisions that others don't agree with, but thankfully they RESPECT my decisions even if they don't agree.

What your niece doesn't realize - each of us has the right to decide what is done with our own bodies. If your sister doesn't want to live in a nursing home - that is her choice. She is not a young child or mentally deficient. She has autonomy.

If the niece takes this personally - that is on her.

However, your sister has to live with the consequences of her decision.

I hope the two reconcile. We can pray for that.