Ok, after giving my theory above, I decided to test it out for myself.
I made a new game and chose Riddle, she was the same as last time, (I got the PI career track wrong, it's Journalism) - lvl 5 writing skill; lvl 2 logic; lvl 4 charisma. She is enemies with Scout and Bobby Sargeant and Wei Keane, and best friends with Justin Kayes. In my game, she did sleep in the child's bed as well.
So, I went to the store bought a time machine, and used it. I also mourned Di. L. Emma's grave and then moved it onto the counter on the left side in the dining room. After making her write a few books over the course of 2 sim weeks, and just 1 sim day after moving the urn, Di. L. Emmas' ghost emerged, so I made them good friends and had them woohoo in the past, twice.
The next day the science opportunity to bring an urn and add a playable ghost to the household popped up, so I took Young Di. L. along. I then made Di. L. (now a playable ghost) enter the time machine and go to the past. I got the following pop up:
"Di. L. Emma went back to see how they became a ghost, but accidentally changed the events so they never became one! Di. L. Emma is now alive again!" :shock:!
PS She is also green :lol:
I have decided that the Riddle girls were lesbians, and that's why Di. L. was in the attic, she was a dirty little secret that the 3 homophobic neighbours never forgave Ms. Riddle for. Now they're reunited, maybe they will start sleeping in the double bed as a couple, (Riddle mourned the loss of her lover and couldn't bear to use their bed at night, so slept in the child's bed instead).