AITA for giving my 8 year old stepsister the rest of my coffee?

I’m 16 years old, I live with my mom. I low key don’t get on with my dad or stepmom so I hardly ever visit. The only reason why I go visit is because I have an 8 year old stepsister who really really loves me.

We are close, I’ll take her to the mall, we buy stuff together, I’ll do her hair etc. Just typical sister shit.

Anyway we were out because she wanted to buy hair accessories and I got myself a coffee from McDonald’s cuz I love their festive menu. My stepsister got a milkshake but then she wanted the rest of my latte so I gave it to her, there was maybe 2/3 left and she drank it.

When we got back to my dads place she was telling my dad and her mom what we ended up doing and going and she said she got to drink coffee.

My stepmom looked unhappy and said ‘uhh, OP you should know better than to give a 8 y/o coffee you should ASK me if she’s allowed to try new stuff before just giving it to her what if she’s allergic’

I said sorry and that I knew my stepsister isn’t allergic to anything, and then my dad told me not to get sassy which wtf, I’m just trying to explain my POV.

When I got home I told my mom and even SHE thought I shouldn’t have given an 8 year old coffee. I honestly didn’t think it was a big deal AITA?