1. The person that crashed into you (sorry, that sucks) could have done the same thing after drinking alcohol. It's legal. Do you also recommend prohibiting it? I think most people would agree driving under the influence of marijuana should be illegal. Punishments should be severe. But even now with prohibition that doesn't stop stupid people doing stupid things. Illegal or not, the world is full of idiots and there's very little we can do about it.
2. I don't mean to be insensitive but your sister-in-law made that decision. It's her right. You have no right to enforce your views on her. I have seen many people addicted to alcohol and it's horrible. But they have an excuse - it's highly addictive. Marijuana isn't. It is less addictive than caffeine meaning that if she is choosing to spend her days on the couch, high, it's a choice she's making.