I have just read through this thread - IMHO, good advice was given and taken, you seem fairly level headed and have thought this through and tried to do your very best to help, you can lead a horse to water .............................
I will say from experince nothing tears family appart like money does or at least the lack of it!! I think you stuck your neck out a long way and went above and beyond in an effort to help!
Of course the downside of reading this is that us being human we now all want/need to know what happens - we are all stuck with the first chapter and I was quite happy not being aware you had a SiL now I need to know if they did/didn't get divorced was someone cheating, did she find somewhere to live etc etc, that's the problem when someone tells a story you need to know the ending!
But whatever happens I wish you and all your family the very best for the future