From what I read about Tony Mirra the guy seemed like one massive liability and Brasco could have been used an excuse to take him out as well. Disgusting that his own cousin had to do him in though...

Welcome to the mob.
Your friend or blood relation can be your best buddy today and tomorrow be the guy who rats you out or whacks you.
Tony Mirra's problem was that he insulted everyone and consquently nobody liked him - Pistone said other mobsters pretended to be his friend cos they feared him.

I dont think Paul Castellano, Carmine Persico or Vinnie the Chin would have tolerated him in their families.
Perhaps thats why the Chin gave permission for the feared wildman Gerard Pappa to be rubbed out?