Sigh They've fucked, or they're currently fucking. Given all the apprehension from both sides, I'd say currently fucking, or at least fucking while you and her were together at some point.

I don't tell this story often as it's pretty hard for me, but if this conversation is going to lead where I think it is, I'd say my story's relevant.

I had a best-friend who I'd known for since childhood. We did everything together and I honestly would've taken a bullet for him. Now for years I was also with someone who I thought would be the woman I ended up marrying. Well I'm gonna skip the gory details, but they were fucking behind my back and ended up spreading some very nasty lies about me to my entire friendship group so they wouldn't look like the bad guys in the situation. I lost everyone and everything and I was depressed for an extremely long time; to this day I have great difficulty trusting people.

You haven't been with this girl for that long, and I don't know if they'd spread lies about you so they didn't look bad, but if your story is going where I think it is, then it relates to mine in one key way: betrayal. Betrayal from those closest to you makes you question everything; there are few emotional pains worse.

If you find out they're currently screwing do yourself a big favor and be done with both of them. Life is too short to have toxic people like that around.

Or hey, maybe it's something totally unrelated like they're actually cousins, pig-farmers or even astronauts! You'll just have to wait and see ¯_(ツ)_/¯