I do not know exactly how “normal” it is for families to cover up such blatant sexual abuse. I know that when sexual abuse does occur in families, it is sometimes the case that at least one other family member knows about it and ignores it for various, complex reasons. In your specific situation, I do think that it is cruel and unusual for parents to witness their daughter being sexually abused by her brother, their son, and to punish the daughter and reward the son. This reaction by your parents is bizarre and, in my opinion admittedly based on very little information, is very abnormal.
I can understand why this situation would bother you but what I am a bit confused about is why this doesn’t bother you more. Your parents witnessed you being sexually abused and allowed it to occur. This should infuriate you. They did not protect you. Perhaps you never learned how to react to such an event appropriately. Using the limited information that I do have from your e-mail, and if I use your parents very abnormal reaction to seeing you being abused as evidence, I think it is plausible to say that you probably never learned how to react appropriately.
I mention your inappropriate reaction to point out that it may be inappropriate and because many people in a similar situation would be upset. Your parents did not protect you and they let you be abused. They let you become a victim. It was very unfair of them and abnormal to let such unconcealed abuse occur to their daughter. I want to also say that if your feelings about the sexual abuse are emerging and becoming too much to handle, you would be very much helped by an empathic and well-skilled therapist trained to deal with sexual abuse issues. Take care.