Usually, people think that men are always involved in smoking. You are clearly not responsible for her smoking. Her smoking habits are clearly not related to you. Your girlfriend smokes because she likes to smoke. It has nothing to do with you or your gender. If her family thinks that you are responsible then make them understand that it’s not your fault. You can’t be blamed just because you are a man. She can take care of herself and should know what’s good for her and what’s not. You don’t have to feel sorry or be accountable for her actions.