AITA for telling my brother he is the biggest imbecile in the whole universe and he needs therapy?

First time posting. English is my 2nd language.

I (20F) have an older brother (23) and a younger brother (14). My older brother, let’s call him Derek, is a self-proclaimed “prankster”. I put it in quotes because frankly I just think he’s a moron who likes to humiliate people and call it a prank. He used to prank me too until I called the police on him once and he’s been avoiding me, calling me a stuck-up bitch.

He hasn’t stopped pranking our younger brother (Jim) who hates the panks and our parents aren’t doing anything about it. They say it’s all in “good fun”. Fuck that. Anyways, yesterday was Jim’s birthday. We threw a party with some friends and relatives. I explicitly told Derek NO PRANKS, not even small ones. He was like, yeah yeah, whatever.

Everyone arrives, we’re having a good time and since Derek seems to be on his best behavior, I’m focused on Jim. Dad brings the cake, we’re all excited. Jim blows the candles (two with his age) and then mom takes off the candles to cut the cake. Before she has the time to do that, Derek rushes behind Jim, grabs his head and SHOVES his face into the cake, hard. It was a fraction of a second, none of us even had the time to react!

The family and friends laughed, but I was mortified. Then I realized Jim was struggling and I helped him get up and wiped his face carefully, starting with his eyes. He hissed in pain and when I looked closer, a piece of cake got stuck to his eye and the area around it was already becoming red. Jim started to cry in pain, I was panicking, mom was panicking. I don’t know what the hell the rest of the family were doing, but I grabbed Jim and said “let’s go to the ER now!”

Then I hear Derek saying something like, “oh there she goes being a stuck-up bitch and exaggerating. He’s fine. Clean it \[the eye\] with some water.”

At this point I was really angry, Jim was scared and trembling, so I told Derek that he’s the biggest imbecile in whole universe and he needs fucking therapy to stop with these stupid pranks. I kinda wanted to keep going, but I left it at that and went to the ER with Jim. He’s all right. They said the cake irritated his left eye and they cleaned him up with a special solution.

When we returned, the party was done, Jim’s friends went home and the family blasted me for what I told Derek in front of everybody. They said it was all in good fun and now Derek is sad and locked himself in the guest room. I said I don’t give a shit and he needs to do something about his immature attitude. He’s the oldest, but acts like a toddler. No, actually I know toddlers who are better behaved than him!

I am STILL getting messages, telling me to apologize to Derek. I refuse to ever do that especially since Jim thanked me for taking care of him and being on his side. Am I really the asshole? I don’t think I am, but I want neutral opinions.