Partnered Sims turn into siblings

I am having this issue. I started a new legacy challenge (PC, no mods, no cheats, all EPs, most stuff packs, a few kits) with a female sim. She and her wife had a science baby. When that baby became an infant, I changed their hair and outfits (normal CAS, not fulleditmode). After that, my sim and her wife showed as sisters. They were still married, had all the other relationship bits, but didn't have romantic actions available.

I searched around and found two suggestions on forums/Reddit, and tried both. I entered CAS fulleditmode and tried setting the relationship from sister to house member. As soon as I swapped Sim or exited CAS, they flipped back to sisters. I then downloaded MCCC and tried deleting the "sibling" relationship bit. They remained sisters. I tried fully deleting the relationships to all other sims - including the child - of my Sim's spouse. As soon as I added one back/introduced them, they flipped to sisters again.

Disheartened, but really wanting to continue this legacy (first time playing the game in ages having lost my mojo), I moved my Sim's wife out. She went on to find new love, and I ignored that her "sister" sometimes knocked on the door. I started to accept this storyline and build around the glitch. Fearful of surprise sisters, my Sim chose to remain only engaged to her new partner. Her partner wanted a child, so they too had a science baby. All was well.

The second child became an infant. I hopped into CAS (no mods, no fulleditmode) to give her some outfits. I closed CAS. My Sim was craving some romantic fulfilment, so I went to flirt and... sisters. Again.

I'm at a loss now, and my mojo for the game is flying out the door again.