Being an "old school" kinda guy, I don't think that simply saving his own ass would have been reason enough for Frankie to flip.

With respect to Frankie's brother, it does not appear as though he was coerced into coming, and he kind of casts Frankie a dirty look when he sees him, rather than a look of fear.

Michael also gives us a clue, if you can take what he says to Kay at face value, when he tells her "It was between the brothers, Kay."