Try the upper floor of her store
Where you will find a key to her private quarters, and with some proper investigation skills (chest) at the "murder scene" and the items you find in her private rooms and "cellar" you will advance the quest further...
The stairs are right to the left when you enter her store, very easy to miss (like all stairs in this game)
Ps.: No idea how to mark spoilers in this forum... but obviously this post in itself is going to be 1 huge spoiler.. so no point talking around the bush...
Also the chest at the murder scene is another one of those "what the heck was Larian thinking" moments, it's occluded by the wall when you enter... like the hatch to her cellar as well (I can't even put in words how much this aggravated me when I was stuck on the same thing)
