I'm sorry that you and your children have health problems - but I don't understand how what you describes relates to my comment?
Of course I understand why some people can't take on caring responsibilities for a niece, but from what the OP describes, it is more of a case of her sister not wanting to do it rather than not being able to.
Of course we only have one side of the story in relation to the OPs situation, which is why my advice to her was about focussing on her niece.
The rest of my comment was aimed at people, particularly those with children, on this thread who were very much in the won't rather than can't category - posters who say they wouldn't do it simply because it would disrupt their own nuclear families, not because they are dealing with their own issues.
It demonstrates a distinct lack of empathy - I would take my nieces and nephews in a heartbeat, because if something happened to me and my DH, I would want our siblings to look after our children, and luckily I know they would.
If I had relatives like the people on this thread who would rather see their nieces or nephews end up in care because it might be a bit inconvenient for them, I would personally want very little to do with them (but I would still be there for their children if it was needed).
Also - you don't know what job I do in children's services, so not sure how you can claim to have trained people to do it 🤔