I'll try to remember as much as possible but can't guarantee everything is accurate

• 2 boys are friends. Both have a sister. Unsure about the older or younger.
• For some reason they want to get revenge on both there sisters
• They end up raping each others sisters (so no incest... yet?)
• They use that as evidence to blackmail them for more sex.
• Eventually there is a scene where they have sex in a classroom
• If i remember correctly it was 4 chapters

Well i've found it

Anette XXX by Ube Yoshiki
https://e-hentai.org/g/483268/8a9fabecd5/