I'm going to use all of the hot water up before my brother takes his shower!
That sounds really mean, why would you use all of the hot water and not leave any for your brother?
Because he took some of my things and claims that he doesn't have them.
It's wrong to intentionally cause your brother to have to take a cold shower. You should consider how you would feel if you only had cold water the next time you take a shower. Instead of using all of the hot water, you should turn off the water when you are soaping up and turn it back on to rinse off.
Why should I care about his needs? He doesn't care about mine.
It sounds like you and your brother have some issues you need to solve. I think you should talk to him and see if you can put your difficulties behind you before it tears your relationship apart.