I (28M) found out that my gf (25F) once had a threesome with her sister (22F) and it creeps me out

I am not really sure exactly what advice I am looking for, but I just want an outside perspective.

Outsider, here.

Should I try to have another conversation with Jess?

To what end? Is there something she could say to make you feel better about this, and if so, what? If you know, then absolutely yes, you should tell her what you need to feel better. (Unless what you want is some huge show of repentance and regret from her. She doesn't have any huge regrets over this, and although it's a bit weird, there's no reason for her to have to feel ashamed of what happened.) If you can't see another conversation changing things, though, then no. It's just going to stir up more bad feelings and potentially cause a fight.

Am I making a bigger deal out of this than I should? To be honest I even found myself considering breaking up over this situation since part of me is afraid I'll never be able to look at Jess the same after this.

That's up to you. I'd say ask for a little distance, and take your time to figure out your feelings. Is there something about this that bothers you other than the "ickiness" factor? I'd say that's a you problem. If you trust her to be a loyal partner, then it's never going to be something that affects you directly. If you think that it has some deeper meaning, like she is an impulsive person that you can't trust, then you should probably leave. Take your time and see what you think when your feelings have settled. Either you'll want to be with her still, so it's good that you didn't blow things up by acting too soon, or you will want to leave, and a few days of waiting won't hurt you much.

/r/relationship_advice Thread