you have to do miras events enough to meet them then after you do the island when you meet them you go to pummu and help shanna then after that you have to find the secret hot spring to get the scene of mira and mirel then you can close the hot spring or not to get to the hot spirng  You have to talk with six npcs: In Aldlyn, Ebron, Parverhill, the Red Keep, Thremten and in Witton. But you must have Mirel in your party already.

In Ebron talk with child, in Witton talk with person near snowman, in Parverhill talk to lonely woman in empty house, in Thremten talk with green-haired man, in Red keep talk with sleepy guard near tent two times , in Aldlyn talk the visitors of the Inn.
It's also necessary that you had the events with Mira and Mirel at the festival of Thremten and that your relationship with them is high enough.
If all conditions are fulfilled you can go to the top right corner of the Calbridge Stronghold, pass the gamble on a small mountain path, and press the action button on the tile directly on the border of the map and after that if you tell the old man to stop the hot spring or not 

Starts either at Pumumu, if you told the old man at the secret hot spring to close the peeping hole, or at the casino in Newkungu after you had the event with Shanna and Naomi at the hot spring.  Bonus scene with her and Naomi possible at the secret hot spring  when you haven't told the old man to close the peeping hole