AITA for sleeping with my fiance's brother?
For privacy, I changed all names.
I (26F) have been engaged to Mark (34M) for 3 months, after dating for 4 years. Mark is the older brother of my two childhood best friends- Mia (26F) and Matteo (26M). Growing up, I rarely saw Mark, because my friends mostly hanged out at my place and even if i went to their house I and Mark never talked due to the age gap.
When I was 19, Matteo and I got tipsy and ended up sleeping together because i was worried about dying as virgin. It wasn't a secret; we told Mia, their parents knew. We tried dating for a week, realized it was too weird, and moved on. It was never a big deal to us.
At 22, I ran into Mark at a club. We had our first real conversation, exchanged numbers, and soon started dating. Early on, the topic of exes came up. I wanted to tell him about Matteo, but Mark cut me off and said, "I don't care who you've been with before, past is past."
Three days ago, we had dinner with Mia and Matteo. Mia got tipsy and said: "I'm the only one out of the three of us you haven't slept with."
Mark was shocked. We explained the past, and he got furious. Mark and i havent talked since that dinner because he said he needs time to think and he is angry at Matteo. I feel terrible that I might ruin their sibling relationship, but I don't feel guilty about the hookup itself. I didn't cheat, and I tried to tell him, but now I wonder if I should have insisted on telling him anyway.
I can't ask my family because they are biased, and my best friends are Mark's siblings, so they are too involved. AITA?