Once you're in Soul Cairn you have to go North East. If you look on your map it shows the major buildings mapped and where she is located is the building in the NE corner of the map with two squares on either side of it. Now keep in mind that she is behind a barrier that you cannot walk through so just get Serena to walk up with you and a prologue between her and her mother will be activated. Maybe it's obvious but you go to the center of this building (in the front because you can't get behind it due to the barrier), walk up the steps and she'll be standing behind the purplish barrier. Hope this helped.