I've tried translating this. It's mostly OK, but I'm not sure what オカズに means in this context. What's for sure is that the little sister masturbated in his brother's room, he sure looks pissed off.

okazu means like snacks,candy, yummies

remember, hideki in chobits hid his okazu in the closet

May be it's too late to respond now three monthes've passed by.
But let me tell you "okazu " means an object about which one think when masturbating.
So the little girl in this picture masturbated in her bro's room thinkin' about his friend.

Maybe hes just pissed because he wasnt the one being masterbated to.