Cannot edit pre-existing Sims?

Did you try the relationship cheats: https://www.reddit.com/r/sims4Cheats/wiki/index#wiki_relationships ?

Are they different age groups ?

Hmm...no, I did not try those cheats. Thanks! 

As for your question, Mortimer and his parents are both elder ghosts but his two brothers are just elders.

EDIT: Okay, that did not seem to work. I'm assuming the < and > isn't required, though. I think I should also note that in the genealogy, Mortimer's mother (not the one I created) is a gray anonymous looking space, despite the fact that I never made that character.

Well, if he already has a mother, he can't have another one. Did you ever give him a mom ? I don't think there is one in the standard game (not sure really)

Yeah, I'm pretty sure there isn't one in the standard game. Whenever I was making his father, it seems to have randomly generated a wife for him that is only visible as a faceless gray person that I am not able to edit. 

I don't know why I was able to make one of his brothers her son though...

I don't know either but it explains why you can't give him another mother.

What about the third son ? Does he have a mother ?

No, the third son does not have a mother.

And does it work to give him one ?

Really don't think there's a solution to this as you don't know the name of the mother of Mortimer.

Are you opposed to mods ? If not, you could try if MC Command Center can help.

All right! Thank you. That worked! 🙂