Since your spouse will not be at the interview but is really looking at the city in terms of a potential place to live, I would suggest that you tell them that your spouse is coming with you, but only to look at the city. Make it clear that you will pay the spouse's airfare (I presume that they are paying yours) and offer to pay any additional hotel fees (my bet is that they won't ask you to - the additional fee for two people is usually nominal). I would also wager that said spouse will be invited to dinner - probably along with spouses of the people that you are interviewing with. This is actually a good way to find out if you are compatible with your potential mentor in a social situation as well as in a scientific one. You will almost certainly be interacting not only with the PI but with the spouse on social occasions - here is a good way to assess the dynamics.
Good fortune to you!