What are the limits of permissible interaction with one's niece's husband?

The husband of one's niece is not a mahram. However, it is forbidden for him to be married to a woman and her paternal aunt or her maternal aunt simultaneously. He is a non-mahram to both of them, so it is not permissible to shake hands with him, be alone with him, or travel with him. He must be treated like any other non-mahram.