Becky isn't stupid. She knows the look on people's faces when she is introduced as Captain Crewe's second daughter, rather than his daughters companion or maidservant.
They look disgusted and some are confused, wondering at how she had risen to such a station. Some of them wonder what had happened for something as unique and strange as this to occur, most of them are just not interested.
Either way, Becky has learnt to simply not care. What does it matter what they think?
The Captain is her father and Sara is her sister, as far as any of the three are concerned.
No one else matters.