I'm controlled by my sibling
In what way does he control you?
I have to do what he says just because he's older than me
There's not really any rule that says you have to do what he says just because he's older. I think you'll be a lot happier if you stick up for yourself sometimes.
I want to but I'm afraid of him because he's bigger too.
Well, I believe you'd be a lot happier if you stood up for yourself. Why not try it and see how it goes?