Names have been changed to protect the innocent.
Jane has type A blood. Her parents both have type O blood.
Anyone who has had a basic biology class knows this is impossible. One or both of her parents must be a type A.
The other siblings have type O. They have never figured out that Jane can't have the same father as they do. (since they definitely know who the mother was)
They have always treated her like a black sheep How would they act if they knew? I don't want to find out.