Partnered Sims turn into siblings

I have a male Sim who is married to Romeo Monty. Prior to now, the two of them were living with my male Sim's twin sister and her husband, Virgilio Capp. After moving Romeo and his husband into their own place, they were suddenly assigned half-brothers. I tried a whole bunch of things to fix it (clearing the saves, verifying files, restoring old saves, deleting the thumb cache, removing and updating mods, etc) but nothing worked. Eventually, I had to use a mod to erase Romeo's family tree and manually re-establish his relationships with all of his family members. Later on in the same play session, Romeo and his husband were reassigned as half-brothers again; I had to use a mod to erase Romeo's family tree and reassign relationships once more. After closing out of the game and come back a few hours later, they're half-brothers again. This does not happen to my Sim's sister and Virgilio.