Thank you so much for your reply!
I tried resetting the sims, saving, and re-entering the household as you suggested, but unfortunately there was no change. Earlier, after I filled the relationship meters by playing, I also tried filling them by cheating, just to see if that was the problem. Even by cheating the meters to full in both friendship and romantic relationship, it doesn't change their relationship status. When I hover over one of them in the relationships menu, it says their relationships are "Despised," "Spouse," and "Best friends."
I have never used any custom content in my game. I did try running a repair on the game anyways, but it hasn't changed anything, unfortunately.
Thank you again for taking the time to reply. I really appreciate it.