<human>: I'm not sure if this is the right place, but I'm desperate. 
My younger brother [12] has been getting into a lot of trouble at school and is all around being a terror. I'm in my 20s away at college, so I'm not around that often, and our mother is a single parent (though she gets a lot of help from my grandparents). I know this isn't "normal teenage" behavior and I'm desperate for some advice. 

All he does all day is play video games, this I know isn't that far from normal. The last few months, his grades have been slipping, so an hour a day time limit was placed on the game systems. Today, when I wouldn't give him the password so he could play longer (I had already given him a 15 minute extension to save the game) he threw a temper tantrum. Not just being upset and crying, an actual temper tantrum. He started screaming and crying and pounding his fists on the floor and hitting his head like a 5 year old. All because he misjudged the time he had left and I wouldn't give him anymore. This behavior is NOT acceptable. This isn't the first time something like this has happened either. 

I'm trying my best to fix his behavior, telling him no at appropriate times, but he just cries and has a fit. I'm meeting a lot of resistance from the rest of my family with the issue, as they see no problem with his behavior and rarely punish him.

Any advice as to what I can do as an older sister to help fix his behavior?

Can you condense the text into three sentences or less?
<bot>: 12 year old boy throws multiple temper tantrums over meaningless things, older sister is sick of it and doesn't know what to do