The MC is actually a girl? I am not sure if it was intentionally kept hidden or if it's just extremely vague writing. Story-wise, it makes no sense as to how she lived with a new family for years and nobody found out that she's a girl; this isn't the 21st century, where everybody has private bathrooms and bedrooms. The other explanation is the family did know, and the author was just intentionally vague, in which case, why? There's literally no reason to do that. That isn't some crazy plot twist; it's just bad writing. Also Feng, the concubine of the emperor and the person they were trying to rescue turns out to be a guy and not a girl, for some reason the author has a fetish for gender subversion. Maybe it's none of the above and the translator just dropped the ball, I don't know. Very confusing.