You are indeed a good brother, OTWO. Like others, I have been in this kind of situation with my mother and another family member.
You are a star.
When I read the bit where you said what was. Really eating you was IF her husband had allowed you to help all those years ago, I thought to myself, no, don't go into What If's...nothing is ever gained from what if's.
But in fact, you then go on to explain just how trickily subtle the problems are, with the way your totally well-intentioned actions could be misinterpreted and made to look bad. I've been in situations like that too, where finding the correct way through an ethical maze where it all matters and one's heart is involved is so complex that it's impossible to find a clear cut right answer.
All you can do is to keep on keeping on trying to the best you can in a most unideal situation.
Thanks so much for sharing it here. I hope venting helped, too.