I'm going to tell my brother I can't assist him with calling into work 'sick' anymore
Why are you refusing to call in sick for him?
It was fun for a while but now it fails to entertain me and I get kind of pissed off.
Your brother should be the one who calls in sick -- assuming he's really sick. It's wrong to habitually call in sick when you're not sick.
He never wants to call because he's lazy.
It is completely fine in this case not to make the calls for him. It is his job so he should be responsible for calling in sick especially if he isn't actually sick.