I've read Casso's book (by Carlo which is doubtful) and I've read the books about the DeMeo crew including his son's. The Gemini twins were pure psychpaths, made of the same material as serial killers like Ted Bundy, etc. their only difference being they were involved in a Mafia context.
I get they didn't have much clout and support but you can't underestimate them.. I honestly think they'd have a fair chance at taking out Casso. It's not like the rest of the family STRONGLY supported keeping somebody like Casso and Amuso in power.