Here are four different posts I've made on this thread about different Sarah activities. I would recommend going through them just to make sure nothing's been missed. After you've for sure followed everything for those, let me know if you still have specific questions / problems progressing and I'll see if I can figure out what's needed next for you. Of course, ignore any "final scene" references if more has been released, because those are no longer the final scenes.
Sarah Pissing and Bathtub Progress:
Sarah's Spa Progress:
Spying on Sarah's Room:
Since I think it might not be covered by the post I'm linking, the MC first has to spy through the keyhole of Sarah's room at night (she needs to have 100% horniness). Then have him go to the PC in his bedroom and do a search (the browser icon on the computer) for spy software. After that, he can find spy software for sale in the store (Other items). From there, I believe this post might help you:
General Post on Repeating Events and How Events Connect to Each Other:
Hopefully this is useful. Most likely there's probably only one thing you missed somewhere; this game is unfortunately like that and can definitely get frustrating if you do it wrong. (I've missed plenty of things myself before and had to keep at it for quite a while to figure them out.)