Sounded familiar. It's a mind control story by Selfavatar.
It contains 32 chapters; starts like this:
A Boy who is now 18. Is adopted, birth mother was friend of adopted mother. Father is unknown?,, Maybe. Has two stepsisters one is about a year younger then him and the other is 9, half his age exactly. Same birth date as well. Father of older sister divorced mom over the adoption of Boy. Younger sisters Father is uncertain though on mothers 27th birthday she got really drunk and kind of slept with boy, older sister knows and had masturbated while watching them do it. Mother does not remember much past being naked with Boy doesn't think a 8 year old could of done anything with her let alone father a child, and she doesn't remember the sex. Boy remembers, feels a little bad about taking advantage of stepmother so has never said anything. Has never considered that he may be the father of his little sister. Mother has not dated since birth of youngest daughter, is horny a lot and has a collection of dildoes and some sex toys and porn hidden in room for release, younger daughter knows about them and though hasn't used the dildoes, "she's saving herself for her brother whom she really likes" looks at the porn a lot and rubs her self but has yet to truly orgasm.
It can be downloaded from Warlord's Yahoo group at this address: https://groups.yahoo.com/neo/groups/warlords_erotic_stories/files
You might have to sign up to download. I don't think I ever finished it.