well, i don't know your stepfather personally, but in most of my experiences with friends' step-parents, they're pretty violent themselves. @_@
um, i digress. if you can, have your older sister explain to him how healthy it is to be vegetarian. see if she'll help you out && such. && just be patient if he still doesn't like the idea. explain to him these are your viewpoints && you're not going to back down on them, && he should be neutral on it, if not supportive.
hope that helps. && if it doesn't, sorry, i came up with this while half-asleep. @_@