In my science fiction story, there are basically two nations that hate each other. There are no contacts between them and they hardly know how life is on the other side of the border.
The protagonist is a young woman who has a twin brother that lives in the other nation. They were separated at birth; they know about each other's existence but they were brought up not to care a lot about it.
Of course, a crucial part of my story requires them to meet. The "government" of both nations would not like it, so there needs to be a very important reason that will convince somebody to help make it happen.
The best idea I can come up with at the moment is a particular genetic disease in the protagonist, that can be cured only with the help of a very close relative. The details about the healing process can be science-fictional, but I would like the disease to be something that exists in real life. Can anybody with a better medical knowledge than mine help?
Edit: I doubt it adds to the question, but since some of you have shown interest, I'll explain very briefly: the two nations are gender based, women on one side and men on the other. Interaction is limited to what is necessary to perpetuate the species... Baby boys are sent on the other side right after birth.