What do you do when your big brother is mean to you?

It very much depends on the behaviour displayed.

If, for example, he insults or otherwise verbally abuses you, you can at first ask him politely to stop. Most of the time, if you respond in the same manner, it will only provoke more abuse.

If he doesn't stop, or if the abuse is not only verbal, but also physical, inform someone with authority of what is going on: your parent(s) should be able to protect you from harm.

Only if the situation is very serious (you have good reason to fear for your health and/or life), call an emergency number (in the US it's 911) or the Domestic Violence Hotline (1-800-799-SAFE).