"I know enough about this matter; let's not tell my brother and Clive for now."
It was about being a child bride, being forced to donate a liver, and almost being sent to someone's bed as an outlet for their release. If Yigol Foster found out, he would be consumed with guilt for not finding his only sister sooner.
Although child brides are not explicitly banned in Yat Nation, they are despised by morality. Even clans with large households would not be foolish enough to publicize having a child bride; at best, they would give her a nominal identity within the household. Presumably, this was also why Daisy Pay was later known as Moon Hawkins as she grew up, but no record of her could be found in the Harrison clan.
The life of Daisy Pay was indeed a tragic one. She hardly remembered anything when she went missing at four years old; by the time she could remember, she was already called Moon Hawkins.