It's never a good idea to ask family members to choose sides...would you choose your brother over your partner if he asked you to dump them? Chances are, no one wins in that situation!
But...if your partner is important to you, you don't want to ignore the disrespect, right?
What i would do (notice i said i), is sit down with your brother and his wife and let them know that you will not tolerate disrespect towards your partner. Let them know that should the disrespect continue, you will have no choice but to visit your family at times that they will not be around. You don't want to lose contact with your brother and nephews, and chances are he doesn't want to lose contact with you either.
