Cannot edit pre-existing Sims?

No, the invisible mother disappeared right after I did that, strangely.

I'm having this problem too. This thread is the closet to my problem. I didn't have enough space to add someone to the family so I wanted to see if I could make it separately and just edit the relationships. Every time I try it won't let me edit one of the relationships in the full edit CAS mode. (So lets say I'm trying to make parents for a sim. I can add one of the relationships, such as "mother" , but father will grey out and default back to roommate.) If I click the check mark and look back at the family tree, I too find an invisible, no named, parent that is stopping the other relationship to be changed. I tried the ID command several times but that does not work for me. I was just wondering if anyone has found a way around this without downloading the MC command Mod.

How is the ID command not working for you ? Are you not getting the ID of the Sim ?

Do you know what the invisible Sim is called ?

I'm also having a problem, has the game been able to fix this somehow?  I have a father and his teenage daughter.

Then the adult father has a son from a teenager

The teenage daughter also has a son from a teenager

The only adult is the father of daughter, all the rest are teens now, but it still registers he's the father, the teenage mother also registers.  Grandfather and nephew are also registering.  The teenage father that isn't being recognized doesn't live with them, why bother if the game doesn't acknowledge it?  Might as well kill the son.

I read that age might be the problem?

It's interesting, the teenage father never showed any bond with his son but bonded with the other child, I'd catch him taking care of the infant, not once did he care about his son, but clearly we know why. Not because their son was opposite of the other and would never shut its yap but the teenage father and his son are only good friends.

I tried to marry the daughter and her fiance', he moved in, hoping it'd fix it, this did not work. 

People say that I can put him in the library (where he will forget all past relationships and he currently lives with 3 of his brothers) and move him in and then I can edit their relationship but after searching, this might not be able to be fixed since the father is still a teen and I don't want him having amnesia. I looked at the Reddit, I already have a mod that I can change the friendship/relationship panel, but why would the boy be sleeping with his dad?  They are already best friends that don't know the truth.  

Do I age up the teenage dad, marry him bringing him in the normal way then demote him back to teen once the game allows me to mark his son as more than a roommate?  Won't it just reset back once I push him back to teen? Full cas mode will only allow father/son to be roommates.  I do not have MCCC, I have tons of mods already and will look into it one day when I'm not pulling my hair out over things as simple as this.