Make alternative plans to do something with your MIL, plans that do not include the brothers. I'm sure she's well aware of what brats they are, and she simply counts on you two to be the better people by ignoring their asinine behavior. Tell her that as much as you love her and want her to be happy, you can no longer deal with or enable their childishness any more and that you want your time spent with her to be pleasant. Continue to call them on their rudeness whenever it happens and eventually they'll either stop the behavior or stop calling period. It's a win/win situation: mom gets lots of positive attention, you get to deal with the losers less, your wife won't be caught in the middle as much, and maybe the brothers will grow the **** up in the process. (I doubt it- people seldom truly change, but it could happen). I've applied all these recommendations to my own interpersonal dealings with family, and it really does work.
