"Not too interested in beasts of the field, but I'll allow this. She's been living with her boyfriend for years, in a D/s relationship. Her slightly older sister who always bullied her as a child has been dating her boyfriend for a few months now, and they are engaged to be married. Already intoxicated by her submissive relationship she couldn't stand up for herself to prevent it. Now that her sister's house has finally sold she was going to finally move in with them, with Sarah as their unpaid live in maid. Sarah had no idea that her boyfriend had met her older sister long before he met her and that her older sister had been coaching him, feeding him her secret fantasies and the right buttons to push to convince Sarah to fall in love with him in a state of abject submission always planning to 'steal' him from her, though he had been in on the plan from the beginning.
Sarah's sister found out she was tied in bed from the webcam and planned for this to happen. She toyed with the idea of 'finding' her sister with her fiance right there behind her - it would be good for him to see her like this as it would emphasize the difference between Sarah and her but decided that it could backfire. He was a bit of a softie and might think this was too far. After all he probably didn't want sloppy seconds from a dog. He still fucked Sarah more than he fucked her though that would change once she moved in.
Had her fiancee checked the webcam since he left her there? I really ought to have, but he was supposedly going to be very busy today. If he hadn't it was a good thing he'd become so callous toward Sarah, if he had, and just left her to her fate, well, that was good too, she thought.
She decided to spin it all as an accident. She would blame her fiancee for leaving Sarah there unattended all day, and promise to tell him all about it. Sarah would be mortified, and beg her not to say anything. She would agree, but then keep bringing it up. Eventually she would get Sarah to admit she liked it even if she didn't gradually pushing her for a repeat performance. By the time she agreed she might suspect this hadn't been an accident, but by then it would be part of her life. Sarah's sister would make it a routine ask and by the time she had her fiancee find out, he would find out it had been a regular thing for quite a while. If he wanted to avoid sloppy seconds from a dog he would have to save his cum for his wife. He would hopefully develop a healthy disdain for Sarah, who would be below Rex in the pecking order in their house. If Sarah wanted her Master's cum she would have to lick it out of her sister's holes if she had anything to say about it."