ok, whenever my brother gets onto the internet from a different computer, and i'm in any FPS game, my ping increases from about 90-110 to 210-230.

He keeps bothering me about saying that he isn't affecting it much. He is also streaming videos off of the other computer.

Is any of this valid to bringing my ping up, or is it another source that makes my ping increase?

Welcome to PC Help Forum

Yup, your brother is using up most of the available bandwidth in your home....this will effect your connection to the 'net..pingtimes, download speeds etc.