From an article at the site www.torah.org there is an explanation of the rule in Judaism against one man marrying sisters.
we see how important it is in the eyes of the Torah for children to get along with one another. The Torah bans two sisters from marrying the same person for the simple reason that the Torah does not want siblings to fight with each other. Whether we are ourselves siblings or whether we are parents who have children who are siblings, we all know that this is indeed a very big challenge.What do you think about this idea?
In the United States and in Israel (and in most modern countries today) no person is allowed to have more than one wife or husband at the same time. In your opinion, why do you think the rules were different in the past? What changed?