Hi 1234...this is just me, but I'd be very concerned about being dragged into a intervention-like posse, as much as my intentions were noble. I've read over and over on this site that it's very important not to get obsessed over the affairs of others, even if they are family.
You say your sister in law hasn't begun her recovery yet. It seems premature to leap in and "help". Perhaps if your brother wants support, you can offer to listen. You can also direct him to Al-Anon since he's going to need that kind of support. Perhaps if you have some literature to lend him, you could give that to him...but aside from that, I'd very careful step aside and let HP take care of him and his family.
Regarding the young child, if he or she is in danger, then perhaps you need to take some kind of action, but if not, as hard as it is, I'd stay out of things.
Again, this is JMO.