While I agree with the advice to LW2, I do not think it is complete. The brother in law sounds a lot like my first stepfather. While the man never broke any bones or left more than a bruise, he liked to rough play. Being left alone with him made me feel helpless, frightened and very degraded. I think people like this get some type of sadistic thrill at having power over someone smaller than them. It is a compulsion and they can't stop. BiL may have made promises, but it sounds like he's a big, idiotic kid. It is up to the real adults in the family to make sure he isn't left alone with any of their children again. Either he'll do something when people are not looking or he'll resort to "Oh, come on. I wasn't even hitting him that hard this time!"

LW1: How does someone else's email "accidentally" get to your inbox? Wouldn't one of the original correspondents need to actually enter your email address and click 'Send?' It's too bad your wife deleted the email. It would be intesting to find out which of your relatives sent it to you. I disagree with Annie here, I think that person wanted you to know what was being said. So go ahead and tell these family members that you received the email and that you want to discuss it.

Re: Datura ---- I absolutely agree with you on this. By allowing this guy to "play" with their children, they're in effect telling their kids that it's okay to play like this. Do they want their kids to have this guy for a role model? And as you pointed out, this guy should never be left alone with the kids; there should ALWAYS be another RESPONSIBLE adult present who will not allow the rough play. Someone has already gotten hurt but he won't be the last if they aren't careful. I can't understand why the lw has put up with this guy's behavior until this point. If it were me, I'd have my eyes on this guy whenever he was in the same room with my kids and there would be no touching allowed.

LW1: Just maybe the mom and sister had some valid concerns about the kids but knew the LW would go ballistic if presented with those concerns in person. Seme people are overly sensitive and defensive about their kids - especially when those kids have real behavioral problems. That could be why his wife just wanted to delete the email and forget about it - even she didn't want to get into it with him.
My BIL and his wife were like that - their kids were rude, out-of-control and little delinquents when they were young. But if anyone even mentioned a concern.....watch out! They'd chew your head off. It was unfortunate because now that the kids are grown, they are uneducated, unskilled and have criminal records that make them totally unemployable. What a waste.