Joanna thought that after helping her get dressed up, it gave Bianca insight into the fact that nothing happened between her and her husband even though they shared a bed the whole night, as Joanna still remembered what her governess told her about pre- and post-marriage consummation to one's body.
Aside from that, by becoming a widow before dedicating herself to de Lara, Joanna believed that Bianca should have known that there was nothing changed in her body that indicated something happened based on the experience she had as a once-married woman and someone's wife.
However, having grown up with the senior maid and interacted with her since she was a kid, Joanna could tell that what Bianca did was not because she did not have insight into what actually happened but because Bianca wanted to make sure that nothing really happened that made her wake up late.
