@Sardonyx409 This issue is marked “needs saves,” so if you’re able to send your save to the devs you might help them fix it. https://answers.ea.com/t5/Bug-Reports/NEEDS-SAVES-Partnered-Sims-turn-into-siblings/m-p/8458978#M113802
(There’s a workaround involving editing your Sims to give them parents that’s worked for some people, but if you can send your save first it would help!)
How do I do that?
It does involve cheating.
On PC, you can open the cheats console with control-shift-c and type testingcheats on and enter. You should get an acknowledgement.
If you have four free household slots you can continue. If not, move one of the partners out.
In the cheats console, type cas.fulleditmode and enter. You should get an acknowledgment.
Edit the household in create-a-sim (plan outfits). If you have Growing Together, do not edit traits! Probably the easiest thing would be to use the play with genetics option to create parents. You can move them out after you create them; they just need to exist.
If you’re able and willing to use mods, people are using MC Command Center and UI Cheats to help fix this. Sacrificial has a mod called Life Manager that might help, as it creates a full family tree for your Sim. Or you can just create parents in CAS.
This video, while not exactly solving your problem, might help. https://youtu.be/mycAKRFS4nQ?si=V0fUcim9lOIqy7gq
