It almost sounds as if the game was programmed at a time when Sims weren't expected to have children, spouses, and extended family members. As if, perhaps, this function was a holdover from the TS4 online days.
However, why on earth would an online game require such a function when there should have been a world of real people, sending real in-game texts to the player? It's just really odd.
The only other assumption that I can make (and, unfortunately, the most likely) is that the game was too rushed to bother with the fine details and nuance of different types of relationships.
