As far as kids go, we have talked about it, but I won't bore you with our discussions too much. Briefly, though, my side of things is that young children need help in making moral decisions. A four-year-old doesn't get to decide for themselves whether they want to go to church; at least until a certain age, it's the parents' responsibility to teach them and make it easy for them to do the right thing, not to give them all the same options as an adult. I'm planning on making it easy for my children (while they're young) to do the right thing by not feeding them meat when I can control it, and also by educating them regarding things like the fact that hamburgers are made of dead cows and chicken nuggets are made of dead chickens (which is something that kids don't always actually figure out until they're much older than I think they should be). Will that be enough to make them lifelong vegetarians? I sure hope so! I think the world would be a better place if everyone (in developed countries, at least) gave up meat, so of course I want to raise my children in a way that I think is moral and makes the world better.
So, wired, I don't know that I'd call your sister insane. Food isn't a moral issue to everyone, but it certainly is to some people. And when something (eating meat, drinking, gambling, premarital sex, whatever) is really and truly a moral issue to you, it's hard to watch the people you love doing the thing you feel strongly is wrong. With most moral issues, I'm usually happy to discuss them, although I generally try not to be the first to bring up the discussion with someone. I'll be honest, though that's as much because I want to make others comfortable as it is because I'm just a chicken about getting into heated discussions. It sounds like your sister is braver about defending her moral stances than I am, which is enviable and admirable in some ways.