Not sure this is a relationship setting bug or something else

@PsychotpsI tried but still shows only wife and husband.. They have only 2 options, household member and wife or husband. That's all. I used relationship destroy cheat and they even don't know each other, and I took them into CAS, but they still have only 2 options. ☹️

This is my female's relationship with household members.

J sim has been her ONLY friend all the time.

G is her ex boyfriend, and R is her current vampire fiancee.

Now the issue is that my female sim cannot be the sisters of Korean named S sim and J sim. Because these 2 S sims were siblings from other Korean simmer's family, so they are named in Korean language. R, G and J are my own sims. I always give English names to my own sims.

J was not her family before but she has only 2 options, household member and husband with J sim too.

Oh I forgot to answer your question. My game is pc version.

generally, sims 4 relationship are a mess if you can’t or want use mods, too limited if you ask me.

I play mostly with vampires, so I want the option that a young adult can be parent from another young adult and why are stepchildren/parents not recognize as family? I don’t like if they can flirt with each other. If they make a generation pack, I hope they rework all family-settings and that sims not can flirt with their great-great-grandparents.

The vanilla way is to save each alone in your gallery and make a new household. They forgot all previous relationships and you can edit it new (Definitely you should use Psychotps method with MCCC)

@aodoherty

You can't just empty the green/red/pink bars. That won't work. If you see a sim on the relationship panel, then they have a relationship. You need to DELETE it completely so you don't see them at all on that panel. Don't even talk to them once it's deleted because that will just create a new relationship.

Just use MCCC to delete it: This is how:

If you want to delete the female sim's relationship with a sim, make sure the female sim is active, click on HER, and select...

MC Command Center \ Relationships \ Delete Relationship

Once the sim window opens up, Make sure you select the sibling that you want to fix. (And any others you want to delete her relationship with) and click the check button.

Once that's done, don't even talk to them! Go straight into CAS and you'll now have the option to set as siblings.

@daikoyu 

You can actually do that. Create a parent/child that are the same age.  It just takes extra steps.

The problem is, CAS won't allow it. So, you have to bypass it.  Pick two sims that are the same age that you want to make parent/child. Make sure they're both in the same household and take them into CAS.  Take the one you want to make the child and lower their age down one age group. (or raise the parent up one) Once they're different ages, make them parent/child. Then just save the household and exit CAS.

Once you get back into Live mode, go into MCCC, click on the sim, go into "MC Command Center / Sim Commands / Set Age"  and set them back to their normal ages.  They will be back as they were, but now with a parent/child relationship.

The other method you mentioned is the only way to set the sibling relationship on console or without MCCC. Save each sim separately to your library as separate households, then create a new empty one and merge the two back into the new one. Their relationship will be deleted and you can set the sibling relationship. It takes longer because you have to go through separating them into separate households, save each to the library... merge them back again... it's a pain. Plus that method has the side-effect of deleting ALL relationships they have with EVERYONE. Not much fun.

MCCC is much easier.

@PsychotpsBefore I take them into cas, they even didn’t have that empty bar. The sim was not on the list. It created the empty bar AFTER TESTING IN CAS. The relationship destroy cheat works in my game. I will prove it  today. MCCC is not my taste, so I don’t want to use it. 

I'm using All Cheats mod in my game, so relationship.destroy 802218617047547946 364243773749920620 cheat is working. After that, the relationship has completely gone as you can see my screenshots.  I didn't know only 3 files are allowed to upload here.  I guess my friend simmer wouldn't want to show her sims all over the world on online. So I'm going to delete these screenshots after solving this issue.

Have your female sim a mother/dad or both? If it so the vanilla game will not allow that you set a sibling relationship, specially not if the other sim have to a mother/dad or both in the family tree. (Please check it in your ingame family tree, the game creates one blank parent if you start your sims as siblings)

You only option here is the vanilla game which she loses all or use psychotps method. You can just download MCCC edit your family and remove the mod after your done. (I had long time ago the same problem, but I used the vanilla method as work-a-round because MCCC clash with many mods from my mod-setting)

@Psychotps, I know this but I will that the game allow it so, without hassle. As I say, the vanilla CAS is to limited specially if you play with aging off or immortal Sims.

@daikoyuThe female S and the male S all have their both parents on family tree but they are remained as empty sims. I think you can be right because Jamie sim is also the 3rd generation, and she can't make him to her brother either. G sim and R sim are all born in CAS, but J sim has empty parents. His parents are actually in my previous save. Yes all age is disabled in my game. I always turn off aging because the default aging system is too short for me.

@Psychotps Well..then I can use MCCC only for this issue if there's only ONE way to solve this issue, then I will uninstall it. Thanks.

I think you will need to remove J Parents if MCCC does it not automatic. As long he has parents you can’t make it to a sibling. It’s by design because you can’t have more as one parents. (Which is logical if you ask me)