Are they adult children, or minors?
The tone of your post is kind of unusual, so I'm not quite sure what to think.
Siblings who choose to have sex with each other should be dealt differently than you deal with things like missing curfew and not trying in school, things that kids usually get grounded for.
Typically, when teenage siblings have sex they are in an extremely turbulent home environment and turn to each other for comfort and stability, however misguided that may be.
It sounds like they both could benefit from counseling. If she's menstruating it's not like she's 4 years old and unable to make this decision.
Best wishes.
The duration of the "grounding" is what you feel is right. You know your son best and how grounding will affect him. Having said that, because this was a "rape", you should seriously consider whether his actions need to be reported to the Police and Social Services. He has violated his sister and her well being is of paramount important. His actions may have already caused physical as well emotional damage to her. She is the one that needs protection.
I would also be very vigilant so as not to allow them to be left alone together without adult supervision.
You do realise that if he did force his sister to have sex with him, this is rape. If he has done this before to his sister or to other young girls, you should involve the authorities (I do appreciate that it is not an easy decision given the fact that they are your children). Depending on their age and mental capacity, you may need to get more specialist help. The sister may also need to have counselling.