Sandra's events step-by-step:
1. Special chest in the swamps near the Bitteroot farm. (Map No. 12) (Nude CG)
2. Talk with Sandra at the sleeping chamber with 60+ relationship. (Oral CG)
3. Constructing the second mine on the overview map will allow you to go to the treasure dungeon with Sandra.
4. A stripping scene starts at Newkungu after having the event with Moriko to find the thieves guild and help their leader with some problems. (Moriko's events begin with a note at the Aldlyn city board after building and upgrading the police headquarters. After purchasing the upgrade you need to leave and re-enter the map to apply the change.)
5. Once you're far enough in the main story to have the mithril pickaxe, a new dialog will unlock for Sandra. She wants to look for Bob, who is at the Wooden House in Begus, where you got the mithril lockpick from Glen.
6. After finishing the events to help the dwarf Bob to find a jade vein, you can talk with Sandra twice at the sleeping chambers to trigger her first sex scene. The scene includes two different CGs with several variations and one animation. (It's not necessary to finish the haunted house events to get this scene.)