It’s about a sister/brother couple that go to a love hotel by accident, however, in order to leave they have to do the deed. They fake it and it allows them to leave, but eventually they ‘accidentally’ return and do it again, this time for real. Otherwise I remember the girl being pretty busty, and she wears lingerie part way through. Plus her hair is mostly black but somewhat blonde too
Edit: Pseudo Sex Siblings for people in need of specifics