Bianca implies she conversed with Isabella alot, she may well have told Isabella about the castle and a trapdoor leading to a subterrean passage to the church.

The old servents' gossip idea !

Bianca chatters of lovers, remarks the servents believe the young peasant contrived Isabella's escape and now pines for a lady.

She does speak as if she has a different understanding of Isabella.
Maybe Isabella confided to Bianca she didn't love Concrad had no wish to marry him.

'She used to indeed to sigh and lift up her eyes in your company, because she knows you are a saint- but when your back was turned- '

Here HW interrupts Bianca's speech with Matilda.