Inviting And Addressing A Christmas Invitatio

I offered to take on the Christmas Eve for the my in-laws this year despite the small size of my home. We had discussed the guest list, but when it actually was given to me in print I was surprised. There was a guest name along with her toddler son to accompany one of the many divorced relatives. There was no last name given. A few dilemmas, in a small home I am trying to keep the numbers down. I am also afraid of too many small children, there are 2 others, in this tiny space, and if I include a guest on one relatives invitation, then that isn`t quite fair for the other 4 divorced relatives with no guest included. I have never met this person or her toddler son. What should I do? Get over it and invite the cousin, his son and their guests???, address all the 5 cousins whom are divorced with their family and guest? Or should I stick to the plan and invite who we orginally talked about and let them rsvp and ask me if they want to bring a guest? There are already 20 people without guests and their extended families.

Thank you for your help?

Sarah