simple as ankmairdor, may have say,that game have a lot of quest that need to be done to a point to the last one, for example the quest involvong the daughter of the chief of the beast men town, must be done before starting the one on the cave, where you will encounter the bad guy, cant remember the name, in order to be finish.
Same for this quest just reload a save before tisha has come get her sister and send her away you will get the next event 100%