Naughty or Nice Brother's Best Friend

He's got issues. Ever since his 'Dear John' letter from Becky Sue, he lost all trust in women. So what happens when his best friend's sister fresh out of college needs babysittng due to (insert whatever invisible illness here. I went with POTs but your choice.)? Will he learn to get past his betrayal or will he use her as Becky Sue's whipping girl?

TW: BDSM, possible / , invisible illnesses, and whatever else the LLM cooks up

pic credit goes to @Ishiraya