I'm (18m) graduating high school at the end of this month. My dad dropped the bomb on me two nights ago that his stepdaughter (14f) has an award ceremony for some competition she entered and won in another state on that same day and that she really wants him to be there. He told me he couldn't possibly make it to both and since his wife and their children together will be going, he needs to be there too. He told me he would make it up to me and we could celebrate another time.

I still live with him (not for much longer). My mom died when I was 7 and my dad got married again when I was 11 or 12. It's been a few years anyway. His stepdaughter never knew her bio dad, so my dad has accepted her as his own. And he has prioritized her a lot in the last 5/6 years. It doesn't always show in the most obvious ways but it can be felt. Father/son time was put on an indefinite hiatus and instead dad told me we needed to include her in our time together but he also spent time with just her for father/daughter time. I brought it up to my dad and he told me I wasn't exactly making an effort to be closer to her so he wanted us all to bond and didn't want me to just focus on my relationship with him.

He has attended her dance things instead of my basketball games if they're on at the same time. It doesn't matter if mine was known about first, he will still skip my stuff to go to hers.

He will take us on family days and whenever he and his wife say "kids can choose" he picks her choices over mine. He claims it's because they will be the most fun for everyone but really, he even says it afterward, anything his little princess wants.

Our refrigerator and our shower broke at the same time. His stepdaughter's birthday was coming up so he took money from my birthday fund to pay for that stuff and so his stepdaughter would definitely get what she wanted (this barbie house thing and a whole fashion set and they were I think dad said $250). He didn't get all the money back by the time my birthday came around so instead he bought me a $30 gift card for Steam when he had promised me a new monitor and keyboard for my computer (that was a gift from my grandparents).

When my dad told me he wouldn't be at my graduation to go and support her, I told him there is no making up for that and he can forget about being included in my life going forward. He told me he would make it up to me and I told him I will always come second to his little princess and I'm not going to be okay with that. I told him he's discarded me for the last time. Dad begged me to be reasonable but I walked away. Then I invited both sets of grandparents who agreed to come. His wife told me I could have come with them and I don't need to attend the ceremony but could support "my sister". I said her daughter's not my sister and I do not want to support their family anymore and I will be out of their hair soon. She called me selfish and told me I can't deny her daughter a dad.

AITA?