When you do picnic with sister in the park on sundays. After a while you can do "special picnic"... After that, you can go into your sisters room after midnight and fuck her. On weekends you can talk to her in the room, and then do the pull tits out (or something), then show her pictures from the picnic and fuck her. She will get mad at you, Give her a magazine, or some perfume and she accept your apology. Now you need to wait a week until next weekend, and you get an event, where you can beat up her boyfriend. Once you do that, she is yours, you can screw her anytime you want kinda.
Now you can do the sleep with mother in the morning at 9, and the sister will bang on the door...