I know that I'm projecting here, but I have dreadful memories of dealing with everything because DH's adult children and grandchild lived in England rather than Scotland. I was too timid to ask them to step up, but I wish that they'd visited when DH was in hospital. (They weren't hard up and it would have been manageable. One of them retired before the age of 50 and money wasn't a problem.)
I've already mentioned the open-heart surgery. When he was later in hospital for 4 months following a stroke, he did get one half hour visit from his son.
If there had been a bit more care shown, it would have meant so much to my husband and it would have helped me a great deal: by the time DH had his stroke, my father was gone, but I was working full-time and dealing with Mum's dementia too.
It was made pretty clear that I was expected to get on with it because I was here (of course) and because I was 'healthy'. In actual fact, I was just about on my knees.
I'm assuming that if the OP's SIL had to ask her sibling to step up she must be at the end of her tether.
I'll just add that my husband's children would no doubt have said at various points that he wasn't ill but recovering, when in actual fact he was a frail elderly man.
The closest that I got to asking for help was when I phoned the daughter to tell her that I was waiting for test results and that her father would need care if anything happened to me (in the sense that someone would have to advocate for him). Her response was "You're not thinking of leaving him are you?"
When I got the all-clear, I phoned to let her know but emphasised that he'd need help if I went before him. Her response was "But you're all right, aren't you?"
I keep thinking that that is the attitude that I see on many threads on here: if one adult child lives near the parent, the thinking is that they can manage because "they're all right". If they're asking for help, then they clearly are not all right.
I'll add that my husband quickly went from someone who was engaged in sports and martial arts to needing me to help him with showering and putting his clothes on in the morning.
His children just seemed to latch onto the fact that he had learned to walk again and ignored it when, for example, he commented that he could no longer put on his socks himself or when I had to cut up his dinner for him. He was compos mentis, therefore he was "fine".