Spoilers up to chapter 35 of the novel

Warning SPOILERS ahead
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Ok read up to chapter 35 in the novel, and the manipulation is big in this one, the sister is just the excuse to get her, he had it all planned out: offer of more money each time she wants to quit the new "task", using progressively worse "tools" so that when given the choice she'd end up chossing the "lesser pain" of getting spanked by his hand, harder lessons on the little sister so that the sister would inevitably end up making more mistakes, and making the FL keep count of the mistakes so that, when the FL predictably got scared at the constantly rising number of mistakes (knowing the punishment would be "1 hit per each 1 mistake") and started lying about number in order to receive fewer hits herself, he could then swoop in to take advantage, reveal that he'd discovered the maid FL's "deceits about the real number of mistakes," and hold it against her in order to force her to choose between two forms of "physical compensation" (under threat of having to return double the money she got in total, if she refused to choose either): first option was more corporal punishment (for her own lies this time, and with worse "tools") and second option was agreeing to a sexual relationship with pseudo BDSM (but not proper BDSM) "lighter" punishments. The Fl ends up agreeing to the "bdsm - ish" (but more forced than bdsm-like) pseudo relationship... and that's when he takes her to the "secret room" and does it to her for the first time, also making her agree to all the forms of spankings that he wants to do (with and without tools) in between...