Everything else I would have said is already in the comments; I don't know the name of it, but the model is something like circles of concern. The person at the center of this (your mom): all of her concerns and worries flow out from her; all of the support flows into her. People in the next circle (close family) have the same sets of arrows - concerns flow out, not in; support flows in to them.

Practically, it means this: if you have someone else that makes themselves the center of attention, make sure they don't make your mom listen to the concerns or troubles they have. Make sure your mom can express her worries or troubles however she wants, even if it's offensive or troublesome for you and others in the inner circle.

My sister is going through cancer right now, and her friends have set up meal trains, transportation to appointments, and the like. It's super helpful for her, spreads the support around, and I think overall a good thing.