It is not a sin to have two or more wives but God doesn't support it. He made only Eve for Adam. Check round the Bible, there is nobody that married more than one wife that didn't have problems. If not in their life time, problems came after their departure. People like Gideon, David, Jacob, Gilead etc. It is better to have one wife than to have many. Because inheritance problems may still occur no matter how you have trained them.