I know we've encountered this problem before, but I can't remember the ruling. Give me a few minutes.
Also, my tip for anyone wanting to prevent this: there is a way to check if a spare's intended spouse lives in town or not. If they live in town, you'll get the option to invite their household over, and their household might include another sim or two, or it could just be them. But you'll get the option regardless. If they're homeless, you don't get the option at all. If they're a service NPC, you can also invite their household over, but when you click on that option, you'll get a very long list of sims.
So see if you can invite their household over, and then see who's in the household.
Now to go to the archives! I'll see what we've said in the past.
EDIT: Looks like Sunshine has had this problem before:
I have a problem my second gen's twin just got married, thinking I would move her out to live with him. Well apparently he has no house and it's not possible to select one for them to move into. What should I do? I've left it paused on the screen but apparently there's no cancelling now they're actually married.
And Metro's ruling was:
Quit...go back to a previous save and do not let the twin get married.