This description is also found at Megilla (13b). How is the betrayal of her husband as well as causing him great embarrassment and to violate halacha (since he was having relations withone woman while he had his mind on another one) - considered praiseworthy and why is it labeled as tznius (modesty)?
Bava Basra(123a):... Wherein did Rachel's modesty lie? It is written, And Jacob told Rachel that he was her father's brother and that he was Rebecca's son. Was he not the son of her father's sister? But he said to her,[Will] you marry me? [And] she replied to him, Yes, but father is a sharper, and you will not he able [to hold your own against] him Wherein, he asked her, does his sharp dealing lie? I have, she said, a sister who is older than I, and he will not allow me to be married before her. I am his brother he said to her, in sharp dealing. But she said to him, may the righteous indulge in sharp dealing? [he replied]. With the pure, [Scripture says], Thou dost show thyself pure, and with the crooked Thou dost show thyself subtle.[Thereupon] he entrusted her [with certain identification] marks. While Leah was being led into [the bridal chamber] she thought, my sister will now be disgraced [and so] she entrusted her [with] these very [marks]. And this accounts for the Scriptural text, And it came to pass in the morning that, behold, it was Leah, which seems to imply that until then she was not Leah! But, [this is the explanation]: On account of the [identification] marks which Jacob had entrusted to Rachel who had entrusted them to Leah, he knew not [who] she [was] until that moment.