That seems to be the most common fight. I don't hear much about father in laws and "insert relative" but always hear about something wrong with the mother in law and daughter in law.
It is a controll issue. The mother will not let go and the wife wants to take over the life of her son. They fight over the guy. The mother in law feels she is older and wants to tell the daughter in law what to do and the wife is not gonna have his motehr interfering in their life together. IT is an age old situation that many people go through.