Many first wives can never let their co-wives be. They are known to attack them at any given opportunity they get. Why can't first wives let their co-wives live in peace? Why can't they just ignore them if co-existing is hard?
Of course many first wives will advance the argument that they own the husband. Now, let it be known that no woman owns a polygamous man. It has never happened and it will never happen. He belongs to all his many wives.