I'm not adding anything of value here, but I just have to say that I agree with what's already been said. People do dumb stuff, but - meh. If he wants to make bad decisions, then that's his right. No matter how much you might want to dictate his life, I would hope he wouldn't really let you do that. I understand you have great intentions, and that's admirable, but I'd find it harder to respect a man who was living his life by his sister in law's rules than I would a man who made bad decisions, but decisions that were his own.
My 40 year old brother often dates women younger than me. (I'm 30) He's also ridiculously immature, and no woman his own age would find anything in common with him. It's what works for him, and that's cool. His business, not mine.