Sims relationships are messed up

[jopeaches]

Hello, I've been having this problem for a while now, but I decided to overlook it since I believed it was a bug or something. I have a household of two sims which are like the third generation of a couple of sims I've created. They are lovers, they are not related since I was able to get them together, but when in CAS mode (with cheat), I can't make them have a kid by playing with genes and that is only available to this certain specific couple. I simply can't create a kid that has the genes of the mother and the father. Also, while I was looking into finding a solution for this problem, I realized that when in cas mode (with cheat) and I click to create a sibling for ANY sim in the game, the game will create you one but I don't have the option of SISTER/BROTHER in the relationship panel. I run out of ideas and I don't know how to solve this problem.

[Psychotps]

@jopeaches The problem is the existing relationship. As long as they currently have ANY relationship, you can't modify it in CAS.

What you'll have to do is remove their existing relationship using MCCC or commands, (MCCC is easier) so they are strangers, then edit the household in CAS and you'll be able to do it then. If you don't want to build up the relationship again, you can always use cheat commands to get it back to where it was.

[EuphorialQueen]

@jopeaches 

Please confirm are you on PC/Mac or Console.
Is this the standard or Legacy Edition of Sims 4?
Do you have mods or cc of any kind in your game?
You say they are "third generation of a couple of sims". Were they siblings at any point?
"I can't make them have a kid by playing with genes" Are the couple in question at least young adult or older?

Testing on a vanilla (no mods/cc) folder is the best way to investigate causes of issues.
The simplest way is to do the following steps:
Rename your The Sims4 folder in Documents\ElectronicArts to The Sims4_Backup
That will preserve all your saves and content so nothing is lost.
Go to Origin launcher and right click on the game and choose the repair option.
Test with a new save game without putting anything back to see if the issues are still there.

Next put a copy of the saves folder (from The Sims4_Backup folder) to see if you can now add the desired relationship.

Source:
Troubleshooting tips
https://answers.ea.com/t5/Technical-Issues-PC/READ-FIRST-Troubleshooting-your-Game/m-p/3621199