Family Kids Only

My fiance has a big family with quite a few kids. With most of them traveling into town, as well as us actually being close to his entire family, we decided to allow only family kids. However, now one of my close friends who lives a few cities away is asking if I can make an exception to the rule but this is not her kid in question. My friend has a medical disorder so she can't drive and her ex husband won't take the day to drive her so she asked a friend to be her plus one. At one point, this friend and her husband tried to become friends with my FH and me but things didn't work out after some uncomfortable comments made towards me by her husband. Because of this I don't feel comfortable around her friend and I am not close enough to feel comfortable to allow her to bring her baby to my wedding. I already told my friend the rule we had set on kids and want her to come to the wedding since she is already missing my bacherlotte party and bridal shower this weekend but I don't want to make this rule exception because then I feel like I need to make it for everyone. How should I respond in the most polite and understanding way possible?