AITA for refusing to bankroll my dad’s “surprise” 60th after my mom called me ungrateful?

I’m 26M. My mom decided my dad “deserves something big” for his 60th and told me I’d be helping. Not like setting up decorations, but paying half the catering and renting a karaoke machine. Around six hundred dollars. I told her I can’t, I just moved into my own place and money’s tight. She said “your father paid for your college, the least you can do is show some love.” That hit a nerve. I told her if she wants to throw a big party, that’s her thing, not mine.

Now she’s telling everyone I don’t care about Dad and that I’m ruining his birthday. My sister texted me saying I’m selfish and that Mom does everything for us. My dad actually told me he doesn’t even want a party, but she won’t hear it. She’s more focused on appearances than what anyone really wants. I’m tired of being guilt-tripped into paying for things I never agreed to. I’m not their wallet.

AITA?