ETA: Title should read AITA for NOT becoming my 'stepsister's' penpal and not trying to have a relationship with her? I only realized my mistake now. Sorry!!

I (17f) live with my dad and he raised me on his own. My mom left my dad when I was 1. She moved out of state and didn't really mother me. For a few years I saw her for two weeks a year when she'd come home to visit family and I'd get some of her time. But really she wasn't a mom in those moments. Just a lady visiting. That stopped when I was around 9 or 10. She's ordered to pay child support but has some debt because she didn't pay for a while. Not sure how much. My dad never told me. I heard it from others in the family. He has always tried to preserve my opinion of my mom where he can. It's a losing battle because of her and not him.

Last year my dad got very sick and I had to move out of state for 6 months to stay with my mom. I didn't want to and I don't think she wanted to. But it was 6 months of no child support.

My mom had another daughter after me who's 10 now. She's been married for a few years to a guy who has a daughter who's 16. I didn't know about her daughter or her stepdaughter when I first went and that was a surprise for me. Her daughter isn't her husband's kid and there was weirdness about that when I was there.

Anyway, my mom's stepdaughter is pretty lonely. She has no friends and no family around her age plus she's an only child so she tried to befriend me and cling to me when I was there. I didn't like it and I tried to be nice but whenever I engaged she got more attached so I ended up staying out of my mom's house most of the time and I'd lock myself in the guest room so this girl couldn't chase after me like a lost puppy. When I went back home after dad got better I didn't make any promises to stay in touch and I sure as hell didn't intend to hear from any of them again.

But my mom reached out and said her stepdaughter really liked me and wanted to be friends and wanted us to be penpals. I found it weird because it's not really the old days where you had to write letters to people to stay in touch. Mom told me it would cost me nothing to say yes but I told her it would never happen. And any time she asked me or dad my answer stayed the same. Then dad got a call from mom's husband a week ago saying his daughter wanted me to come for her birthday in June. I said I didn't want to go and dad passed that word on.

Mom got mad and said her stepdaughter was upset about it. She liked me and wanted to be friends and sisters even. Even though I know mom has another kid and a stepkid I still say I'm an only child and I have no interest in a relationship with either girl. I told mom that. She said I have no reason to say no and to think of how much it would mean to her stepdaughter and then she cursed at me for not trying to have any relationship when her stepdaughter wants it.

AITA?