My sister's husband always compares her looks, cooking, bringing up kids with me and his sister and thinks that we are better. This breaks her heart. She supports my BIL financially and works hard the whole day yet he wants a "perfect" wife. How does one make him realize her worth?

This is truly appalling, and I've personally see this issue with a friend's family. The only thing that came to my friend's rescue (who was constantly compared with other women in the family) was herself. She got fed up of her husband comparing her to others and I don't know how she gained the strength, but she finally stood upto him and asked him to stop comparing. She told him that his words were hurtful and how she was contributing equally to their relationship and even taking care of his parents, while he would go out of the city for days on end. The good thing is that she instantly got her MIL's support. In this situation also I would advise the same. Nobody can help you, but yourself. You can only say as much, and you should as well. Correct your BIL when he compares his wife to others. Similarly, your sister should also stop him when he does it to her directly. Trust me, after she corrects him a few times, he won't repeat this again.