Her quest is kinda buggy but there are work arounds. Here is a good heads up, everytime you see an exclamation mark on top of your companions, talk to Sebille first and soon, don't wait. This will be crucial in order to advance her quest when you start Act 3 (before leaving the ship for the Nameless Isle - you'll know it when you get there). Otherwise the quest can bug out and you'll have to cheese it to complete it. It's a bit far off, but keep it in mind.
