Ok, so that was a weird moment.
Diane was very annoying, being that I only started her once everyone else was complete, figuring the Therapist I had impersonated might have thrown the cat among the pidgeons.
So out of spite, I would always have a session with Veronica before going over there, making sure I was getting as many shags in to spite her as possible between sessions.
Before the 'final session', I made Veronica clean the kitchen floor and treated her to a good shag before I went over for the trap with Mom.
Because the scene isn't a long one, once me and Mom returned looking thoroughly shagged, my slave big sister was still on her knees in the maid outfit sans panties cleaning the kitchen floor our Mom has been praising me for cleaning each time while I chatted idly to Mom about our incestuous sexual relationship not nine feet from her.
I almost wanna reload a save and see what happens if I picked vacuuming the floor instead, that way she'd be in the back of the very shot in which Mom and I are talking. It was priceless.
If only the game could acknowledge coincidental moments like that as I've experienced a few fun moments like that while grinding through the back nine of this game. Good overall, but the final few achievements really put the screws to my patience vs my completionist drive.