I actually edited their relationships with "remove relationship bit". I had to do this with my sims father in-law, wife, and children only. After that it said the children were her step-children and her wife was her cousin instead of sister, but the family tree was intact.
August 2020 - last edited August 2020
I noticed that this bug is more likely to happen to Sims who don't have parents or to Sims with parents who are the founders of a family and who are married to a Sim without parents. For example, Grandma and Grandpa Marchant are the founders of the Marchant line. They have no parents in-game, and they don't show as siblings. However, both of their daughters married husbands without parents, and their relationships keep switching to siblings. None of the married relationships in the following generation were affected, regardless of whether or not the Sims were married to spouses who had parents. This bug also affects sibling relationships in another way: Full siblings become half siblings - each sibling "loses" one parent. This bug happened in both the second and third generations.
I can temporarily fix this with MCCC and another mod that allows me to add rel_bits and calculated relationships, such as Aunt and Uncle. I've also noticed that this bug happens when you try to add Parenthood rel_bits to Sims, such as Oh Captain, My Captain. Additionally, this isn't the first time the Devs have broken family trees and Parenthood rel_bits.
I have the exact same problem. Editing relationships using MCCC and/or shift-clicking and removing the relationship bit both fix the family tree, but romantic interactions between my married sims are gone and I haven't found any way to get them back.
I'm an occult player, and i've been using the husband1 spellcaster to wreck havoc everywhere, and ofcourse i tried out the duplicato spell. I believe it's the reason why my married sims (with three kids) have become brothers. Tried all suggested fixes, worked for 5 min, then reverts back to being brothers. Sucks.
Ive been having this issue as well.
I only had the problem when (or right before) snowy escape was added to my game. I looked at the new sentiments and i saw the "sister" tag next to my sims' wife's name. very upsetting. it kept their relationship otherwise intact (except it wouldn't allow them any romantic interactions) and their children's family tree was an incestuous mess but accurate otherwise.
I'm fairly good using MCC so i of course attempted many relationship settings, even going to far as to completely clear a generations worth of relationships and family ties in my save and even after all that i would reload the game and it would STILL have them set as sisters no matter what. even if i just left them as strangers they'd end up sisters the next time the game loaded. no other mods, ctrl c cheats, or family overhauls can fix this glitch for me once its been activated. my games desperate to have these lesbians be sisters if its the last thing it does.
my current strategy is to just wait until the glitch is fixed (as i cannot even begin to play this game without regular cas interventions), but I'm open to suggestions to fixing it! (yes I've tried no mods, only mcc in game, everything BUT mcc in game, reset, reloading old saves, and simply not playing the family)
Wanted to reach out and get further information on this. First, we have had a new update yesterday for everyone running into the issue can you first confirm this is still happening? secondly, in order to get some more specific information can you also reply with answers to the following questions (for those who have not submitted a bug report already)?
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December 2020 - last edited December 2020
Yes, I can confirm this is still happening after the update. I downloaded the update today and then used Origin to "Repair" my Sims 4 installation, but my spouse is still showing as my brother. I loaded up the save from before the game decided to mess up their relationship, and if I go into CAS by clicking on the mirror, once I come back out, the game has added the sibling bit again. So even if I went back to that point, it's still doing it, and I will just never be able to use CAS without suffering this consequence. Honestly, I would not want to go back to this old save, even if EA fixed CAS at some point, I would lose too much progress. I'm really just about ready to hang it up, at least with this family, which is my favorite family. Please help.
I'm on a Windows 10 PC. This first starting happening just randomly after I went into CAS one day to pick a new outfit. I have never used CC or mods in the past, nor have I ever used cheats. After this very issue started, I used cheats solely to try to fix it, but to no avail. I have the Realm of Magic game pack, and, yes, I have used the Duplicato spell with two of my family members. That is the only cloning I have done. Please help, I'm really sick of trying to fix this with no results.
Edit: I'm not sure if it helps, but I wanted to add that this family has very long-lived members. It's a spellcaster family, and they use Potion of Rejuvenation, which has resulted in grandchildren being in the same age bracket as grandparents. The grandparents are the ones with the spouse/sibling bug; both of them have used Duplicato on themselves.