My wife always compares my parenting with her sister's husband's parenting and thinks he is a much better father than me. I find this comparison annoying and heartbreaking. How do I make her understand this?

That is wrong from her side...Yet it is perfectly normal and understandable. We always try to imperfect our parenting skills. She usually does this because there are certain aspects she requires your help in. Try to figure these out and be supportive. Be open to her on what you are willing or not willing to to, and explain thar your circumstances like work, time etc.. is not the same as other parent and that she should not compare you. Try to slip for her a little flattery now and then in the form of " you are better mom than ur sister " ( hence need fewer support from me)