I was stuck at the "sex scene with sis" part, but somehow things worked out for me. What I did was to make some things the walkthrough wasn't telling. I peeked on her when studying on the couch, then I noticed I couldn't skip the dialogue, as if it was new. Mby she changed her clothes or something? And I was caught by mom once. I tried to spy on her in a different time (19pm i think?), then she caught me. That's about it! My route was entirely dedicated to sis, but I did agree with mom when she asked for my help. I didn't give her the cards or anything, it was another kind of help that she wanted. And then I finished the day like any other, went to bed, gave sis goodnight and the scene triggered when the MC went to sleep.
BTW, something important to tell about this game is that it's
NOT WORTH YOUR TIME ! the game is not made for a quick fun. it has too much grinding and only a few scenes... doesn't feel rewarding enough. the CG isn't even good... this game got famous, but to me it's one of the worst experiences I had. six hours for like, what? at one point u get a hardon that lasts for 2mins, and another u end up doing quests for 20. how am I supposed to enjoy this.