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

Possibly. I would have to understand my motivation in revealing it.

You don’t want to get involved in that stuff.

In both cases i would not tell their wife/husband, i would only tell them that i saw them kissing somebody else (or at least not hide away when i saw them kissing).
What they are going to do with that situation regarding their SO’s is up to them.

If they were happily married & had no outward signs of a strain on their marriage i’d pull him up on it & ask him what the fuck he was thinking of.I wouldn’t however tell his Mrs,that would be his prerogative.Sort yourself out man.

I think I would slink away and not say anything to his SO until after I had a good chance to talk to my brother. I would never rat out my brother.

So long as it’s none of my business they’re off the hook unless I’m asked directly about it. If they make it my business (lie for us, help us, etc.) or are on my bad side, they’re in deep shit. In any case I will make it clear to them I know.

I would tell my sibling if I saw them misbehaving. I would advise them to smarten up and avoid ruining their relationship/marriage and hurting their S/O. I would not tell the S/O the first time I caught them. If this kept happening and I caught them again, I would demand that they confess to their S/O before I felt obliged to talk to them myself. I would expect better conduct from my sibling.

If it was the S/O that I caught, they’d get the demand to confess to my sibling before I told them myself. Blood is thicker than water but not as much as people say.

No. If I had a brother and we were close, I’d confront him. If he were to be cheating I would think he was having problems.

I would ask them wtf they’re doing cause they’re in a load of chin high shit!!!!!!!

Seriously.

And yes I would tell, because if it were me being cheated on, I would want some one to tell me too.

I would want someone to tell me if I were, in the words of Linkin Park “Like a chump, like a chump, like a chump.” I think that I would confront my bro. Probably his wife too. I’d then expect her to tell him to save me the trouble.

@Glow Speaking of loads of chin high shit…

Disclaimer: The above link is totally unrelated to this question. It just seemed appropriate in light of @Glow‘s comment.

I would if I was a private detective and hired to do surveillance on him by the sister-in-law.

I would confront my sibling, but not tell his/her spouse.

Unless I knew my brother/sister had an open marriage, I would tell my brother/sister that they need to talk to their SO, and that if they don’t, I will.

Unless I knew my brother/sister’s SO had an open marriage with my brother/sister, I would tell my brother/sister’s SO that they need to talk to my brother/sister, and that if they don’t, I will.

Blood makes no difference on whether or not I would speak up, as it seems to for a lot of people here.

Nope I’d keep my nose out of their private life.

After reading the responses and thinking about it more, I’ve realized that if I rat out my brother, he’ll be pissed at me for years, maybe longer, and never really think I’ve got his back anymore. His SO may or may not stick around.

If their relationship ends, I won’t ever see her again. I’ll see my brother for the rest of my life.

If I rat out the SO, I’ve got my brother’s back and she may or may not go away. If I don’t rat out the SO, I know my brother is being made a fool and I’ll resent her for life.

In either scenario, my relationship with my brother is more important, so I won’t let him look foolish, nor will I ruin his marriage against his will.

I would never rat out my sibling. I don’t care how much I liked their SO.
On the other hand…if I caught my sibling’s SO, then I would approach them and give them the ultimatum of fessing up or I will.

Sorry, family has its perks.

Blood is thicker than water. Your loyalty is to your brother, who is blood, rather than to his wife, who just married in to the family.

The only times this won’t be true is if the person really doesn’t like their brother, or prefers the brothers wife to the brother, or if you are so squeaky clean moral that you can never do something that is unfair or unethical in your view.

I wouldn’t say anything.

If my brother ever did this I’d confront him, ask him what the hell he thinks he’s doing- then offer to set him up with a good marriage therapist.

If my Sis-in-Law (any of them) did that then the same goes for them. Family isn’t just blood.

I’m with @rebbel – I would tell my brother about what I saw, the rest is up to him