The best way is cold treatment. The more anxious you are to explain and emphasize something now, the more afraid and disgusted she will be.

You have several advantages. Your sister-in-law should be on your side, and someone will help you speak. What you need is to do your own thing first, then you need to know what she likes and what her hobbies are. Just learn from the side. When you are ready, let your sister-in-law try to create opportunities for you to get close to her.