Can someone please tell me if I made a mistake?

I’m in a group chat with my girlfriend of 2 years and our best friend. I was making really stupid and funny tik toks that were only being shared between the three of us. I decided to make a very stupid, non serous joke about sleeping with my girlfriends brother on the last Tik Tok I made.

It was a stupid audio and a weird filter where I first said “Me and my girlfriend vibing to Minecraft music” where the next part was the filter where it makes two more of you (so basically there’s three of you) and it said “The leftover sperm in my uterus from a few hours ago left by her brother”. It was completely a joke and she knows I’d never actually try anything with her brother, for one, I’m not into guys and two, I hate her brother because he’s literally the worst male I think I’ve ever met. And now she’s really upset at me?

I thought it would be funny and I apologized but she won’t stop being upset about it. I explained to her it was just dark humor and I would never mean any of it and now she’s still upset. I don’t really know what it’s like to have older siblings and siblings even close to my age, so maybe I just can’t really see where she’s coming from? Did I cross the line from an honest perspective? We’re the kind of group to make gross and messed up jokes like that so I thought she wouldn’t really mind it. But I could honestly be wrong.

Also P.S, this has been basically my first serous relationship and I still have a lot to learn, I also haven’t had a lot of good examples when it comes to healthy relationships so I could be totally way over the line with this one. I just truthfully don’t know and I want to learn from my honest mistakes. Thank you.