MC/Al was a girl by birth, but they stepmom didn't want Al to became candidate for their house succession so Al was raised into a boy. Even going as far to fed him with poison of infertility. But after being healed, her menstruation circle come back (yes, there a panel showing how panicking she was as never been thought about menstruation) and she start to embrace her feminine side while maintaining the boy-ist appearances. Although some people already catch up that Al was too beautiful as a man
As for succession, I think it was mainly the man who inherited the title but there also case a noble (acknowledge by the royal) may choose the matrilineally inheritance system, even in some case was using merit system so both male/female heir had opportunity to be the heir. From what I see is that the grandfather family was choosing the successor either by matrilineal way or merit system, that's why it look confusing.
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And for why Al was scared to reveal her gender was because all of family member (outside the trashy one) know Al as a male not as a female, and Al herself has been raised and brainwashed through life that she was a male, so she must act and think like one also she never thinking her own body was a body of female (this was one of solid reasonable explenation why she get scared the shit out when find out her first menstruation, like a clueless kid). This is just show how bad the situation itself without explicitly showing it.
Havent read this for the longest time and continued on the chapter i left it on now im confused bc i cant remember mc's fcking gender