Cant edit sim relationships even with cas.fulleditmode

I tried to post this to the sims forums but apparently new members can no longer make posts..

so a sim who had twin boys died shortly after they were born so I moved the twins to another household for another sim to raise.. fast forward and now she (the adoptive mother) is elderly and will probably die in a few days and I’ve only just realised that since her sons aren’t her biological kids nor were they adopted through the in-game adoption process, they’re not part of the family tree. I was gonna use cas.fulleditmode to change the relationship from ‘household members’ to ‘mother/son’ but the only option I have is ‘husband/wife’.

so basically. I have a mother and her adopted sons. The official relationship between them just ‘household members’ but using cas.fulleditmode the only option I have to change it to is to make them married..there’s no options to make them ‘mother/son’. Any help? 

UPDATE

i realised that the biological father (who is just a random townie) is still alive, just in another household so I added him to the household so I could edit his relationship from father to household member and the option to change it from father to household member is there and I’m able to select it (as in its highlighted, not greyed out) but when I do click on it, it doesn’t change. I tried to edit his relationship with both his biological sons and every time I clicked on ‘household member’ it wouldn’t change..