They can stay there or you can move them out to live with another sim (become good friends and get the option to move in) or move them into an existing empty house as explained in this post: -sims-freeplay-empty-house-options/
They can stay there or you can move them out to live with another sim (become good friends and get the option to move in) or move them into an existing empty house as explained in this post: -sims-freeplay-empty-house-options/