What should I do my ex-husband who is living currently with me currently wants to move in his niece without my permission?

If you own or rent the home that you are occupying and your former husband is simply living with you where he has no ownership in the home occupied or is a tenant under an oral or written lease of the unit, he really has no legal right to have his niece move into the premises without your or the landlord's permission.

I would sit down with your former husband to discuss the situation and if he persists in having his niece move in knowing that you do not want it to happen, depending upon if he is not on the lease or an owner of the premises, I would serve him with a 30 day notice to vacate the premises in addition to the niece. You might also consider consulting with a landlord tenant attorney about the circumstances if the niece actually moves in.