We've started leaving the bedroom door open (Sadie's room) when we are home lately, to see what would happen. The boys come and go, but Lucy generally does not enter.
I walked in yesterday afternoon and was surprised to see Sadie on the bed with the door open! She also poked her head out of the bedroom and into the room next door (the former Lucy's room).
After many weeks of no progress, we're pleased that she's finally making baby steps.
Sadie on the bed (with Ringo)!