Is her fertility high?
Anyway, I will look if anything can cause this bug. thanks for letting me know
if you finished all the Sophia's content, until that scene where you make the "deal" with her and you get the notification saying Bianca can now get pregnant you should be able to trigger her pregnancy.
If this is really not happening it may be a bug, if you can send me a save file where you finished tghe game on discord: tenmo_farmguy