hi, as i read in devlog, to unlock this scene is i must complete gambling event, having mithirl lockpick & pickaxe, and "previous event".

what supposed "previous event" to be? is it when open chest that trigger scene sandra with plant? 

i complete gambling event & have those mithirl lockpick and pickaxe, but nothing happen when i talk to sandra in chamber, can somebody tell me what i miss?

The previous event with Sandra is her "treasure dungeon".  To unlock it you need to build the second mine on the country overview map.