J.B.
Ummm, if I am around a seven year old who is cussing and being/talking about sex, then I will correct them and tell them that's not allowed. I wouldn't stand for it. I would start by nicely letting the SIL know that her son was speaking in a HIGHLY inappropriate manner, so she should be watchful of that behavior. If she does nothing say, "You know____, people really shouldn't use cuss words, and kids just aren't allowed to at all, so you cannot say those words -at least around me and my kids." I would say the same thing about sex stuff. Tell him that (whatever he's saying) is for adults and it's a private matter that is not discussed in public and certainly not discussed in public by 7 year olds!