Can you mention to the relatives who do the comparing, quietly and in private, that you don't think it is right or fair to compare the children? Tell them how much you love the cousins and how everyone is different?
I would also pick a few key phrases to repeat to the kids if they are doing the comparing and just keep saying them over and over until they get the picture that your family doesn't do competition like that.
There are always going to be people who are jealous for any number of reasons (ask me about the new minivan my cousin just got for free because her father "had an extra."
) but we can't change them we can only try to react appropriately.
