My boyfriends Dad & my sister are dating

I found out by accident too. I was over for dinner one night & I was telling a story about my family when he said "Well Mel seems to think that.." I hadn't even realised until Ethan (my bf) stopped him and said "Wait what? Mel? What are you doing talking to Mel?" and he admitted that they'd been 'spending some time together' (whatever that means 😳) over the past few months. 

I'm not hating on the relationship or anything I just find it weird.

Anyway, how would you feel if your FIL became your brother in law? Or have you ever been a similar situation?