What is the best medication for fear?

Can you be more specific.

Does she suffer from phobias? these can lead to anxiety and depression like social phobia, agoraphobia (going out in public) and generally all phobias limit a person into leading a "normal" life.

Also anxiety, is a mental condition which can cause fear, tension, worry.
Does your sister suffer from anxiety?

How long was she on this antidepressant for?

With the limited information provided, I believe your sister must see a specialist, a Psychiatrist.

I would really love to help more but I mean this in a good way... I can not be guessing in order to give you an accurate answer, now, if you can provide with more information as I asked before also her age, how long has she had this fear, fear of what, there are many variables here which can affect your sister, but again, I can not guess, so I believe a professional must evaluate your sister and then treat her.

I am here to help and I wish you and your sister all the best, masso