Minglan and Sister Rong were both born as concubines. I think Minglan actually carefully "raised" Sister Rong, including how tired she would be if Sister Rong really wanted to marry into a high family, but she avoided it for Sister Rong.
Sister Rong's biological mother, Zhu Manniang, was the woman who wanted to put Minglan's own son Tuan Geer to death. It was not easy for Minglan to still have the magnanimity to treat Sister Rong well.
As for the faint sense of politeness and boundary, and the grievance of not recognizing Sister Rong as the legitimate daughter, it may come from Mrs. Sheng's praise, or from Gu Tingye's tacit approval, or from Minglan's own decision... But what does it matter?
I don't think Sister Rong really cares about this. Being comfortable with each other is far more important than just showing off.
