When I have to have this conversation, I couch it in a statement that is all about better service for the guest. Example:
Me: Oh, I thought there were only 3 people coming. There are 4?
Guest: Yes, my cousin decided to join us last-minute.
Me: We'll have to update the reservation on Airbnb then. There will be an additional fee of $15/night.
Guest: Do we have to? She's going to share the queen bed with me, it won't be any extra work.
Me: This is for your protection, as well as mine. Suppose there's some kind of problem with your staying here, and you have to file a complaint with Airbnb. Now, I will do everything possible to make sure your stay is good, but things happen. And if the complaint has an extra person not accounted for, you won't be able to complain.
OR
Me: Oh, the house isn't set up for 4 people! Only for 3. Your stay will be a lot more comfortable if the house is set up for all the people that will be here. I will need to call my housekeeper to come and set it up for 4, and I have to pay for that. So I will need to ask you to pay for the extra person.
OR
Me: During the cleanup afterward, my housekeeper charges me per person that stayed. So I will need to charge you for the extra person.
My housekeeper doesn't charge me per person, but the guest doesn't need to know that. 🙂 And I have never had a guest give me a bad review because of this. I tell them, after the transaction is done, that this will not affect the review *I* give them, and when they see that I'm not angry, they're relieved, if anything.