Well you hadn't even met her when he slept with your fiancée, so he didn't do anything wrong there. If he's sleeping with your fiancée's sister that's her business, but if that means he's cheating on his partner without his partner's knowledge, that would be a little problematic.
the past is the past man, but the sister thing.... idk man sounds shady.
Oh no. This does not sound good to be honest with you.
You might tell the mother she might next if she should drop her keys around this guy maybe she should kick them all the way to the car