this story has 21 chapters, and 4 main characters, one is Tim and Susan, I think, who are siblings, and the main character and her sister, forgot their names, Tim and MC are best friends and started masturbating together at 14, MC did some experiment with sister, some shenanigans left Tim asleep and MC with Susan, MC used to stole panties and Susan said she still wears them, later on Susan said Tim fucked his sister, then after that event they went on and did it as a group, stopped there ch12
I know the story you mean, but the version I know is not on SOL. This is one of the old BBS or ASSTR stories that exists in a few different versions by different authors.
The version I know, which I think is the first, is titled "Kidcest" by Steve Jensen. There's a sequel story titled "Family Fun". In the Steve Jensen version the protagonist and his best friend are Steve and Scott (15) while their sisters are Cindy and Susan (13). I found an archived discussion from 1992 which listed the story, so it's at least that old.
However, I know that I've read variations that start with the first few chapters then head off in other directions. It's probably one of those that you saw.
Edit: I found a partial copy of the version with Tim as the name, it was attributed to Steve Jenson and confirmed a BBS origin, but posted to usenet in 1999. It also adjusted the ages so that the girls were 12 and almost 12.
It might be overkill, but there's a link on reddit with a magnet link (torrent) to an archive of ASSTR up to 2017. Not sure if you want to download a 10GB file on the off chance that you can find a variant version of the story, but if you do: r/DHExchange/comments/7fl535/s_entire_archive_of_asstrorg_ftp_site_up_to_may/