The man in question, his father was the former leader of the sect. The former sect leader was a ... less than honorable man. His ... activities shall we say were well-known, but not spoken about in public forum because of his sect's power and wealth.
The man himself was an illegitimate son of his father so he was not the rightful heir of the sect by birth, but he became the only option. His marriage was arranged to the daughter of another sect allied with his own. Before his wedding night, however, he was informed that this young woman was actually his half-sister, her mother having been a less than willing partner of the former sect leader. This woman's mother had kept it hidden because it would have destroyed their alliance with the stronger sect.
Knowing that he was arranged to marry his own half-sister, he chose to not break the engagement and married her anyway. That is perhaps the primary reason.
[cw: mentions of non-consensual and incest] Sorry, it gets really dark in their series
The man himself was an illegitimate son of his father so he was not the rightful heir of the sect by birth, but he became the only option. His marriage was arranged to the daughter of another sect allied with his own. Before his wedding night, however, he was informed that this young woman was actually his half-sister, her mother having been a less than willing partner of the former sect leader. This woman's mother had kept it hidden because it would have destroyed their alliance with the stronger sect.
Knowing that he was arranged to marry his own half-sister, he chose to not break the engagement and married her anyway. That is perhaps the primary reason.