Editing Sims Relationship

Okay. Fiddling around some more. I can make his sister his sister and when I undo that relationship it allows me to make his sons his brothers... ??

@Chrissie1185 
First let me confirm that you used CAS.fulleditmode as the correct cheat code.
Were / are all the family members part of the same household and living on the same lot?
From the way you worded your inquiry it sounds like he was living on another lot.
I think the relationship cheat was among a number of cheats that were officially disabled.
Historically certain aspects {like relationships} of preexisting Sims that is already in the game cannot edited to avoid potential corruption.
The related Sims will need to be part of the same household (living together on same lot) for this cheat to (hopefully) work.
Try entering the CAS.fulleditmode cheat before entering CAS.
Then try to adjust the relationships.
If that still does not work there are mods like "CAS FullEditMode always on".
A lot of players use MC Command Center for this and other options.

Disclaimer: By personal choice I do not use mods.
You will need to check with the mod creator if you decide to use them.
The link below has helpful information you may find interesting.
[FAQ] How to use Mods/CC
https://answers.ea.com/t5/Technical-Issues-PC/FAQ-How-to-use-Mods-CC/td-p/8781409

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@Chrissie1185 wrote:

Okay. Fiddling around some more. I can make his sister his sister and when I undo that relationship it allows me to make his sons his brothers... ??

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There will need to be an appropriate age difference to apply the desired relationships.  If the Sons are young adults their parent will need to be adult or older. Game logic says a young adult cannot be the child of a young adult.  They could in real life but not in the game if the parent is added after the fact.

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@AndyZhang2896 wrote:
@Chrissie1185Can you check your other sims (the one that is still alive) genealogy? If Sarah's mother dies, you can't add his mother to the game and edit their relationship because Sarah already has a mother in the game. The cheat is actually cas.fulleditmode in case you type it wrong.
I hope I'm clear here.

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This probably is my issue. I am essentially replacing my sim with himself but from a previous save file and the game thinks they are two different sims (well they are so the game isn’t wrong) so his sons have a father but it’s his future self and dead.

I am so gutted. I was playing another family and he was a vampire and in the park, during the day, I left as soon as I knew he was there because I didn’t have control over him and he died without me seeing. 

I saw online that if there is no urn there is no way of getting his ghost. Is this true? 

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@AndyZhang2896 wrote:
@Chrissie1185I don't know whether or not a vampire will leave urn after death since I didn't like occult in TS4.
My solution is saving your current household (his son's family) and then put it back to the game again. This will delete the son's genealogy so you can't see the son having a father in the family tree.
In this way, you probably can add the sims back to the son's household and edit him (the vampire) as the father again.

But you will risk the son's career I guess, skill level and stats still intact though.

*Remember to back-up your file, I'm afraid if something goes wrong. Remember the cheat is cas.fulleditmode

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Thank you so much. That worked!

@Chrissie1185  @AndyZhang2896 
Very happy you worked out a solution.
Happy Simming ❤️