Not only is he an asshole, he's a moron. She has no reason to tell on him, and it's pretty obvious that she's a straightforward person so if she says she's not going to she probably won't. And he has to know that she's his sis-in-law's best friend--a close connection of long standing. So if he tries to mess with that it's going to be tricky, and obviously if she finds out she's going to be pissed and more likely to say things he doesn't want said, and if sis-in-law feels he's trying to sabotage that relationship she's going to be upset with him. There's just obviously a huge chance of it backfiring if he tries to bust them up like that--he's a fucking idiot.
The only kinds of person who would do that is the kind so used to deception and manipulation that they've forgotten how to interact with people normally, or the kind who have such a hardon for causing suffering that they do it even when it's bad for them. I think he's the first kind. Plus he might be such a habitual liar that he finds it hard to remember that some people aren't.