On the "elevetorlevel" it was simple, here I'm just not able to "activate" "her", I've tried a lot. Thanks anyway...
Just watched a vid, to refresh my memory.
The stairs fold out automatically after playing
the right notes, you just turn the cross to make
the stairs stay in place. You do so by clicking the
cross right after playing the notes.
Bertha seems to be auto-controled so you
probably don't need to switch to her.
If it still won't work then we may be talking about
a bug here. Didn't beat the whole game on android
so I don't have anything else to suggest