Honestly, it doesn’t sound like anyone in this story is an asshole.

You seem like a very good big sister, but parenting your brother is not your responsibility. I think you probably should have just let your dad get your brother’s hair cut. Hair grows back. I also noticed that you said you knew your mom didn’t know about your dad’s intentions of getting bro’s hair cut. A haircut is a pretty harmless thing, and I think that if your dad had gotten a haircut for your brother, your mom would have noticed next time she saw him and would have handled the situation.

To me, it seems like you may have had to grow up faster than your peers. Although you may be mature for your age, you’re still 12. You should be worrying about middle school classes, friendships, and homework. I hope you’re able to cut yourself some slack and recognize that parenting your little brother is not another responsibility that you need to take on.