My sister (25F) is spending Christmas with my best friend (26M) and his kids. How can I stop being mad about their serious relationship?

I can definitely say I don't have any deeper feelings for Andrew. He's like a brother to me, in fact - he IS my brother. That's how close we are together. I can admit, I don't entirely know why I'm feeling how I'm feeling. At first I was very happy for their relationship, but seeing how serious it actually turned out to be - it upset me. I started to feel uncomfortable about Andrew (someone I think highly of) being with my sister in a serious way. I'm not entirely sure why.

When he asked me if I would be okay with my sister and him dating, I was happy! But at that time, I didn't expect things to become as serious as it got. Andrew's kid calling her mom, our whole family referring to his kids as my sister's "girls" and them now moving in together to be a family - I don't know why, but all of this upset me.

I wish there's was more I can tell you, but like i said - even I don't understand why I'm upset. I feel ridiculous having all these feelings.

/r/relationships Thread Parent