I've (17m) known my best friend since I was 3 when he and his family moved next door to me and my mom. My mom and his mom became close and they'd have regular playdates for us. We've been inseparable since then and we spend so much time together we're like brothers really. We're both only children but we've acted like siblings and his parents and my mom always treated us like brothers when they realized we felt that way.

My dad never really did but he was always fine with my best friend coming to his house when I'm there. My parents are broken up so I spend some time with dad but I mostly live with my mom. There was never any real problem with this before last month.

Last month was my best friends birthday and I wished him a happy birthday on social media and called him the brother I never had. I've said it before and probably even posted it before. So has he. But my dad's girlfriend was pissed off that I called him that when her son (14) is there. She said I have a brother now. I admit I rolled my eyes at that. They've dated for 3 years but I've only known them a few months and she doesn't fully live with dad yet. But even if she it wouldn't change anything. He could become my stepbrother eventually but he still won't be my brother or be someone I'm close to and our relationship will never be what it's like between my best friend and me. Anyway, sorry, she didn't like me rolling my eyes and she told me I should be more careful with her son's feelings because imagine how he'd feel if he had social media and saw it. I told her that wouldn't be my problem and that I've known my best friend since we were 3. She told me it doesn't matter and he can never really be family. Then she told me to invest in a better relationship with her son.

At that point I was over it so I left and I told my dad I was staying with mom. But I did go back and his girlfriend came over while I was there again and she told dad and me we were doing a family day. My dad was all for it but I said I wasn't and that I was leaving. She said I still needed to apologize for the post and she was giving me the chance. I told her I wasn't going to apologize.

Apparently she's still pissed about my post and my refusal to apologize and my refusal to be her son's brother.

AITA?