Sweet Valley Twins!!
Sweet Valley Twins and all of its variations was another series that I loved as a kid, but not as much as the BSC. The BSC was my go-to series that had a ton of books and a million spin offs, but I did enjoy the Sweet Valley Twins books a lot as well.
I didn't read too much of the Sweet Valley High series. At that point, I think I'd grown out of them, plus the high school series seemed really dated. I do remember reading one about Elizabeth getting into a motorcycle accident and being in a coma throughout the book; at that point, it was like, really? A coma? After all these Wakefields have been through so far?
There was ALSO a Sweet Valley University spin off series, but I'm not sure how many books were in that. I remember looking at one at a grocery store, which featured the twins on the cover in very revealing bikinis. I specifically remember being embarrassed looking at the cover, wondering how Elizabeth could WEAR that. In my mind, she was still this brainy little writer who would never dream of showing cleavage!! (Obviously, I am over that crisis now.) I never read any of the University series, mostly in fear that my parents would see the book and be like, what is this pornography???
The Sweet Valley books, finally, introduced me to the idea of ghost writers. During my SV heyday in late elementary, I remember wondering how Francine Pascal wrote ALL those books and consequently learned that she didn't. It blew my mind that she was credited for each book as the author on the cover, even though she was just the creator. Verrrry interesting to tween Kyle.