My SIL has made it clear that she does not want to do 'care' for her mum. She 'helps' by ringing other family members to:
a) point out safety issues and repairs that need doing
b) critique the home improvements we have organised
b) ask for updates about medical issues so she can explain these to us (because we don't understand medical things and could be putting her mum's life at risk by only seeing horses where there might be zebras).

SIL also rings her mum, but this is sporadic. She will ring her daily when her own life is steady, but if there's lots going on in her home then she might not ring that week. Therefore MIL cannot remember any phone calls with her as there is no pattern to it.

SIL usually visits her mum once a week. Usually on her way back from shopping, or collecting a child, so it is late afternoon or early evening when MIL starts to sundown. SIL usually brings her own meal with her to eat in her mum's house because meals are a social activity and it is nice for her mum to watch her eat (apparently).

Sometimes SIL and her kids come and do a full scale cooking session, e.g. Sunday dinner, or baking, and leave all the washing up for her mum - to give her something to do (which means that another family member has to do it).

SIL believes that her mother-daughter relationship is more important than mother-son or MIL-DIL relationships. Therefore her needs as the daughter must take priority over ours - we must enable her mum to have the best relationship with her daughter and her daughter's children. In her mind there is a pecking order, with men and in-laws coming last. We are the functionaries that enable the quality emotional relationships to flourish. I kid you not.

SIL will not commit to any arrangements or future dates because something else might crop up in her own life that is a higher priority. To us this is both unreliable and selfish behaviour, to SIL it is a practical approach to managing family life as a single parent. SIL has a number of health problems and has come through several years of tremendous personal, emotional and physical stress. Taking on any care for her mum, or commitments to appointments etc. would add to her stress levels and impede her slow recovery [true fact, I'm not being sarcastic].

From the way I have written this, you might think SIL is a selfish c*w. Or you might think she is doing the best she can (but could try harder). Or you might think that hers is a highly sensible and pragmatic approach. It's all a matter of perspective.

When I get resentful towards her I have to remind myself that I have no right to judge her, or anyone else. If she was more charming and gracious she would probably get away with her low level of support. As it is, she is grumpy, aggressive and whiney, which makes it harder. There is an issue of input, and there is an issue of attitude. It is difficult to separate the two. If you can do this, things become much clearer.