Partnered Sims turn into siblings

I have been playing a polyamorous household with multiple women forming a family and having children together. Today as I was playing, I noticed two of them could no longer have romantic interactions together. I tried checking their relationship to see if their dynamic had shifted, and it said that they were sisters. One of these sims is one I created, and the other is a base game townie. The two of them have literally had a child - and their son does list both of them as parents - and not even as a science baby, just traditional woohoo. They were not married (each of them were married to other women in the household (for money)) so maybe being in the same household together confused the game?

From what I've seen in other bug reports, this tends to happen with same-gender sims and I have yet to see any reports of this happening to different-gender couples. I'm not sure what issue is happening on the back end with two sim women being in love, but I'd hate for a game that prides itself so much on love equality to continually have a bug turning lovers into siblings.

I have no mods and have no interest in them as I don't think my computer can run them, I'd just like to figure out how to make them unrelated and in love again. Their son needs his two moms! Not sisterwives!