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?

Maybe she should try and tell him that why doesn't he himself tries to do everything the way he likes it alongwith all the things your sister already does and then find out that not every person is the same! He may then realize that things are always easy to say and not everything is just a womans job. Everything nowadays can be handled by both the genders. Tell her to be confident and stand up for herself.