after reading your post on the other page and this one here, sounds like you miss a step somewhere and you can't get it to trigger until you do it, what step, we don't know, there is a few of them you have to do for it to trigger i think, only thing i can say is, get the walkthrough, look at each step from the start and see if you done that step until you get to the one that you haven't done yet, if you have the walkthrough and going by it, you must of over looked a step, or got ahead by a few steps and skipped over it, it is easy to get ahead and skip a few steps and not knowing it like it did, i also had to go back and see what i skipped over, i also somehow skipped ahead a few steps as well and didn't even know until i got there, and the way to trigger the threesome scene, catch mom at the pool in the chair, go close to her, rub her feet, move her leg, etc, up to the point where you get a bj from her, then you sis will catch the 2 of you, ask her to join.
The first time your sis will walk away and you will have to talk to her about what she saw, then talk to mom after, I also think you have to do the anal scene with our sis at the pool at night, catch you sis bf with your neighbor emma and show your sis the video, that's all i can think of that might be part of it to get it to trigger.