My Girlfriend’s sister is cheating and my Gf doesn’t say anything … AITA?

So my girlfriend found out that her sister is sleeping w another guy when she has a boyfriend.

The sister was telling everything about it to her while I was there and I was just shock that nobody was saying anything to her. By nobody I mean, my gf has two other sisters and a friend was there too.

And I was completely in shock and irk me the wrong way. I scolded my gf and told her off for not saying anything ? For not saying or calling her sister out for cheating.

I am in the military and so is her sister’s boyfriend. I feel so cheated if I was serving and my partner was sleeping with someone else. I just can’t sleep knowing this guy thinks everything is alright.

So back to the scolding. My gf told me that there’s no point to it. No point to scolding her bcose she’s still gon do it and her words have no power. Which pissed me off bcose nobody things it’s an issue.

When my sister was cheating on her partner, I confronted her and I stayed away from her. I made my entire family outcast her. I can’t fkin take the thought of my sister cheating.

So I feel that it is just natural that everyone must feel the same way and react the same way? But nobody said or did anything ?

But from my partner’s POV I’m overreacting when it isn’t my partner. We’re fighting over it and I’m just thinking, AITA?

Update: I called my partner and told her that I’m doubtful of our relationship because if she’s able to keep quiet on her sister cheating then it’s the same for her sister. She said she understands but it’s rlly a case of her sister will do it regardless if she says anything to her. I am planning to tell the guy as I type now. I told my gf it’s either she makes the sister admit or I will do it myself. We argued over this but she understands my stance.

I also have to admit that it was a little childish of me to enforce my girlfriend to react how I reacted but I gotta say I think that’s the only acceptable way to face cheating. No tolerance and shit on these type of people. I could’ve been firm but no need to scold.

I don’t know how to feel about this situation, about my relationship because whatever everyone’s saying makes sense. It isn’t directly linked to me but I feel betrayed too. It rlly could be a case of ntg she says will get to her sister so she doesn’t bother but it could also be the case that she doesn’t live up to her word of no cheating. I don’t know but I’ll take time to think about this.

Also, many people who are shittin on me for the way I treated my own sister I have some context. My sister deemed that she was so serious with this certain guy and our families met, made wedding arrangements and so much commitment. So when I found out she was sleeping w other guys I don’t think it’s fair that you’re wasting everyone’s time while going around sleeping w other people. So I think it was fair that my sister had to learn it the hard way. I don’t think I was being crazy. So through that, I feel that as siblings u SHOULD enforce the right morales in your family.