Apologies... There's not enough information to
Enable me to feel comfortable with offering unconditional support...
Could your friend either be innocently mistaken? Or a **** stirrer?
Surely... If you think it's a possibility that your BIL is going to try and cut you out... There has to be something that has happened that you're not telling us? Are your parents quite critical of him? Has there been a conflict?
Is he really in a bad way or do you just have differing standards of what is acceptable? Does he have a few beers a night in which case I would say ease up butt out and give him some support as he has been through hell... Or is he drinking a carton every couple of days and leaves the kids unfed and dirty?
Good on you and your mum for helping out with the kids etc... But please don't turn and twist that to say he's incapable.
This man has lost his wife my heart breaks for him. My condolences to yourself and your family as well - so heartbreaking.