The list of those a man may not marry begins with Mother, Daughter, Father’s mother…Son’s daughter, etc. and continues down a long list that includes such unlikely situations as marriage to one’s Daughter’s son’s wife
The entries pertinent to my research were:
Why did it matter? The hero of The Forgotten Daughter is the half-brother of the heroine’s half brother. But she is not his father’s daughter and not his mother’s daughter. He is not her father’s son and not her mother’s son. They share neither parent and were not raised in the same household. We’re good. I know you are all relieved.
You can find it here: