If you do any genealogy you will find cases where the birth certificate lists another name, frequently the brother. Women were shunned when they got pregnant especially if it was a powerful or important person. The girl would put her brother down as the father but the DNA shows it really was the master of the house where she was working.
Yes, sometimes it is the older brother who takes advantage of the younger sister. I've known cases of older brothers paying for therapy years later for their sisters. The psychological trauma is even worse when it's the father.