Yep, and nothing happened
Your question does not make sense, there is no "slaves bar". There is a slave list on the right side of the mansion screen, but there is no id code that can get Emily back into the player's slave list. If you sent Emily away with Tisha and gave them extra supplies when they left, then she will automatically return after a few days.
If she left under any other circumstances, then she will most likely have been erased from the game so you cannot get her back. At that point you can only either try again during the next game or edit the save to start the quest over from the beginning. The save file is a plain text file and the folder can be accessed through the mansion's save/load menu. Make a backup copy of the file before editing as mistakes are hard to fix. It will have a value called "sidequests" and shortly after that will be a pair of values like:
"emily":100
Change the 100(or whatever value you have) to a zero and save the text file. Emily can now be found in Wimborn again.
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%