If you're in a relationship where you are pursuing sexy times with the other person, you have implicit consent. I've had multiple dozens of instances in my life where my partner or just a close friend woke me up in this manner. I've had multiple dozens of times where I did the same for my partner or friend.
There was never a single time either side would even TOLERATE someone else considering it rape, much less considering it so ourselves.
You're a f'n brain dead imbecile if you feel the need to vilify that scene as both the MC and his sis had already had clear intentions with each other.