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?

When he starts the comparison, interrupt his conversation and change the subject. If he still continues, then at least you stand in support of ur sister. Tell all present what a wonderful person she is and how she supports. If he nudges on further then say it out loud that he's a thankless man and such talk is not welcome. At least ur sister will be happy. And really...kudos to her for supporting a such a MCP