People are Telling Me (25M) That my GF (25F) Cheated on Me?

SOLVED

So last month, my (25M) girlfriend (25F) of two years got asked to go on a cheap vacation with a group of people she kind of knows. She’s friends with two people in the group, and they were going to get a discount if they had ten people to go on the trip. It was a 10-for-8 discount and basically, two people would go on the trip for free. But their friend group had only nine people, so they asked my GF.

She didn’t want to go but her friend pressured her, saying that she’s never left the country before, and it would be fun. She caved and went on the trip. And she hated it. Like, she would call me to do nothing but complain about the activities and the group. Which I understood because the planned trip was all stuff my girlfriend hates. Scuba diving (She’s afraid of the ocean), hiking for a day and a half (She doesn’t like camping), going to parties (She has sensory issues), “being a slut on the beach”, which is what she called the day she spent watching the other nine girls flirt with everything that moved, and other stuff. She’s introverted, and I can count on my hands the number of times she leaves the house in a week, including taking out the trash or going grocery shopping. She’s WFH as well, so she doesn’t have to go to the office.

She came back about 2 weeks ago, and the day before yesterday I got a text from my sister (26F) saying that we needed to talk. I met her yesterday, and she told me that my girlfriend cheated on me on the trip. Apparently, one of the girls felt guilty and told my sister, who’s friends with her. According to this girl, my GF went clubbing with them, had ten shots, got drunk, kissed a guy, went to the bathroom with him, did a line of coke off his dick, and then proceeded to have sex with at least a dozen different men.

And guess what? My sister apparently got video evidence of this! She showed me a crappy video in 360p of a girl with the same hair color and build as my GF riding some black dude while blowing and jacking off 3 others and saying that they were so much better than her boyfriend. It was like watching some weird cuckold porn, and I was mystified. The quality of the video was so horrible that for all I knew, it could be Lana Rhoades in it. And none of the actions of that night described my GF.

1. The clubbing - She hates the sensory overload from the loud music and flashing lights. She would never go to a nightclub.
2. Ten shots - My GF is a lightweight that can barely function after three cocktails. If she had ten shots, she’d probably be dead. Not to mention that she hates the taste of alcohol and only drinks fruity cocktails.
3. Coke - So apparently my GF got blackout drunk and also decided to do hard drugs?? That makes no sense to me.
4. Sex with a dozen men - She’s still a little nervous when having vanilla sex with me, so unless the liquid courage and coke did a 180 to her personality, she would never agree to a gangbang.

And this apparently took place on their last night there. Wouldn’t she have a horrible hangover, not to mention whatever the equivalent of a hangover for coke? When I picked her up from the airport, she was so fresh and happy to see me. In contrast, all the other girls looked like zombies.

There were more holes in this story than swiss cheese, but my sister was so insistent that I just told her I’d talk to my GF to get her to leave me alone.

I went home and said that people said you cheated on me, and I’m confused why, and she was just as confused. She said none of that ever happened, and she spent the last night there in her room on her laptop. She wanted to get a good night's sleep before the flight, since she finds it hard to sleep on airplanes. And one other important factor is that she was cheated on in her previous relationship, and she told me a long time ago that she would break up with me instead of cheating because no one deserves to go through something like that.

I believe my GF, but it’s my sister who’s telling me this. Someone I’m incredibly close to and trust. If her friend came to me with this story, I likely would have laughed in her face. And honestly, it took everything in me to not laugh at my sister because the story she gave me reads like a crappy porn plot. But my sister wouldn’t lie to me, so now I’m conflicted. And now she’s gone and told a bunch of our mutual friends, who are also telling me that the “proof” all points to her cheating on me. The proof is so full of holes that no one would ever believe it, but the person telling me this is one of the most important people in my life. But another person incredibly important to me is telling me that the story isn’t true.

What the heck do I do?

Edit: So she lied to me.

She didn't spend the last night reading AO3, she fucking binge watched the Mandalorian, even though we were supposed to watch it together >:(

And also a bunch of random cooking YouTube videos.

I had stern words with her, and I'm rethinking this relationship after her shady behavior, because I can't be with someone who's so heartless and inconsiderate about my feelings. Her deceit has hurt me so much that I'm actually considering breaking up

/s (It's a joke lol)

I hugged her and we ate dinner, and now we're trying to figure out why someone would try to destroy our relationship.

Also I want to find out if my sister is just gullible, or if her actions were malicious.

Thanks u/Lichenbruten for the history idea, idk why I didn't think about it lol