Hm, she doesn't have a a phantom mother so you should be able to change it.
Does it work if you do it for a new version of her via the gallery ?
CamtheWolfGamer What are the ages of the sims involved? If both are in the same life stage (Baby, Infant, Toddler, Child, Teen, Young Adult, Adult, Elder), CAS will not let you make them mother and daughter even with the cheat on. The one you want to make the daughter needs to be younger than the mother. Example. If both are young adults, then you need to change one of them to either Adult or Teen. Then you can set the relationship. You can then try setting the age back to what it was on the sim you changed but sometimes that wipes out the relationship again. Not always though.
However, if you use mods, you can do it directly in live mode with MCCC without going to CAS without having to change their live stage.
Hope this helps.
It didn't but it did give me an idea
