While that situation would be inappropriate, the game doesn’t prevent it because it would restrict story lines for players. An example would be that some people like to make orphanages or apartments, and have many unrelated Sims occupy the same lot. It would be good if the player could simply add a tag, to prevent the situation you described though. I know there is an option to “adopt as caregiver” (think it’s from parenthood), but I’ve never checked to see if using that would prevent your issue from occurring.
