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?

you are the one who will have to take the stand for her and show her true worth to him and the world. if he can't already see that he has the 'perfect' wife, then you will have to point it in his eye and show him. the moment he starts all this, ask him to stop and counter his conversation, telling him with proof what an amazing person, wife and mother your sis is. do this in front of her kids, so that they too realize that their mother is truly amazing and respect her more.