Is this an irrational fear?

In the end I would say go with your gut. Is your fear irrational? I dont think so. My moms biggest fear in the world is that one day her cat will walk past a candle get caught on fire and continue to set the house on fire... my point is that although these are all possibilities you have to judge all variables in your particular situation before you make a decision. its possible that your step brothers will hurt your cat but maybe having this conversation with them and expressing your concern will give you a better idea of what you should do.