@RandomBuzziness I agree on the reading thing. Even though I've been playing this game since day 1, I am still finding things I didn't know about too.
To answer your question on MCCC. When I tried to use it, it wouldn't let me have two mothers either. I did basically the same thing that CAS did. I could only end up designating one mother. At first, it wouldn't even give a choice to choose a 2nd mother, only a sister. When I used it to delete all relationships with the child, after I created the 1st mother relationship, then it did give me that choice again but as soon as a created the 2nd mother relationship with the child, it deleted the first so in the end, it wouldn't do it either. The family tree ended up showing a mother and that empty portrait you get when the game doesn't have a second parent.
Yeah, MCCC is really good for that. I use it a lot for certain things. I was looking for one of the maxis sims that shows up in one the pack demo videos in the gallery and I came across a bunch of Sims 2 sims they had made including the Pleasants. Daniel and Mary Sue and teenage versions of Lilith and Angela. So since DU gave us YA versions of the girls, I added them to my game, deleted the teen versions of the girls and used MCCC to set all the family relationships and now have the entire family in my game living in the Brichester house.
Did the same thing with the Fengs a while ago. In an earlier save, I had just left them as not played NPCs and because I like MCCC's story progression, they had 6 kids, 5 girls and a boy, who by the time I stopped playing that save, had aged up to teens. In my current save, I've played the Fengs a lot (you would not believe what I've done to them <grin>) and even had 4 more girls who are now grown and married and got kids of their own so I went back to my library and added the teens back in and reset all the relationships and the family tree and rel panel looks like they've been there all the time. Why I mention it is that I did the same thing you have and there is a Feng family club as well as a few others like it for some of main families. I also use a mod that allows up to 20 sims in a club and it works really well for the most part. Their gatherings can really get hectic with the whole bunch in one place at one time though. But it's fun.