It's tricky, and it's been a while, so my memory is hazy at best. I don't remember the exact choices, but the main ones were when you're with the maid and sis is coming to bring you a gift. If you sleep with the maid, sis goes into yandere mode, I think that prevents the scene. Also, after the meeting(do not go home with yakuza girl), you either have to choose to sleep with Mom, or sleep in your own room, I forget which, but the next day you'll either share a shower or a bath with sis. Bath scene is the one you want. If you did everything right, you'll tell her about your time in the military, and flashback to MC deflowering his friend, who is also an LI in the present, only if you got this scene.