Is it possible to have sex with the sister if yes then how
Yes,
Route 1
1st talk to her about her boyfriend,
2nd meet her boyfriend at the community center
3rd agree to let her have your room for an hour
4th spy on them
5th go to bed horny
6th touch her while she sleeps
7th go to bed horny
8th touch her while she sleeps more
Route 2 (possible bug as I think this route should only be available after the above route)
1st meet her at the community center on a Friday and get invited to a party
2nd drink with her
3rd dance with her
4th dance topless with her
Possible route 3 (not sure if it works)
1st go to bed horny
2nd get spotted masturbating
3rd talk to her about it
4th go to bed horny
5th touch her while she sleeps
6th go to bed horny
7th touch her while she sleeps more