My brother is in the sixth grade, but he acts like he's in second. Why?

I'm in the ninth grade. My brother, as mentioned above, is in sixth.
Whenever I talk to him alone, he's nice and we make a bit of conversation. When my mom enters the room, he hollers, "SHE HIT ME!" and I try to defend myself by saying I didn't, but my mother never believes me.
One time, I was super stressed for an exam, and I asked my mom for a glass of water, since her arm was near the pitcher on the table. My brother snapped, "Can't you do it yourself?"
I was shocked; I just asked for a glass of water, but there he was, getting FED BY HAND, and he's implying that I'm lazy. So, as I said, I was stressed, so I snapped back, "Why can't you just shut up?" That was it - no swearing or name calling.
He got so angry, got up, walked around the table, and punched me, hard, in the arm, and my mother had nothing to say, except, "Don't hit your sister."
Why did that happen?