Would you tell your sister-in-law if you saw your brother making out with someone?

This is purely hypothetical. Imagine you are at a party and wander off to take a piss, or throw up or whatever. Off in some dark area you stumble across your brother/sister making out with someone from the party. Your brother is married. How would you address this situation? Slink off silently and pretend you saw nothing? Ask him what he’s doing? Tell his wife?

Now consider the flipside: you see your brother/sister’s SO making out with someone. What do you do then?

I think I would tell my brother if I saw his wife making out with someone else, but would not tell my brother’s wife if I saw him making out with someone. I can’t fairly justify this position.