I've never quite figured out why any guy would WANT more than one wife! That means you'd have more than one MOTHER-IN-LAW!..........AAAAA-r-r-r-r-r-g-g-g-g-g-g-h-h-h-h--h!.....
It would be hard to treat more than one equally to the others. I guess if you got in an argument with one, you could just go to bed with a different one that night! But, think of the jealosy it would breed in the house, and what if they were all mad at you? sounds like a nightmare! When I was in Saudi I asked some of the TCN's if their Islamic law allowed them to have more than one wife, and most answered, "Why would you want more than one?"
You are wrong, If you marry the wifes sister you only have the one mother in law, then if she is a widow you can marry her and not have any.