More details are needed. If the stepsister can rewind time and force you to complete a challenge, why can't she rewind time to stop your parents from catching you doing naughty things with her? Does she already have a love nest prepared, so the moment your parents throw you out, she can swoop in and have you as a prisoner in its sex dungeon, forced to spend your every waking hour tending to her?
Are there other girls you can hookup with, and how do you stop the stepsister from going yandere and killing these girls? Maybe she made a deal with the devil to get her superpowers, and a term of the deal is she must refrain from killing her romantic rivals out of sheer jealousy? Will the devil be a girl you can hookup with, allowing you to turn the tables on your stepsister, and make her a prisoner in the sex dungeon she prepared for you (though I doubt she'll resist much in that case)?