Justin.tv was part of the summer 2007 y-combinator batch (with dropbox and disqus among others). Twitch branched off of justin.tv a few years ago (I think technically part of the same legal entity at the moment but I'm not sure- they were trying to separate the infrastructures etc. when I left). Emmett, the CEO of Twitch, is also a partner at Y-combinator if that hasn't changed.

Twitch is (was) Justin.tv which is a YC company.

They spun off of Justin.tv, which was in YC summer 07.