The first thing I asked myself is, 'what is scary about a witch?' What still has the power to frighten and unsettle? Definitely not the 'old hag' version that we are overly familiar with from Halloween images.
When I was a girl, witches were a big part of my life. I fancied myself a witch, and had a best friend who was my witch sister. We had names, artifacts, a family history, and we spent many hours inhabiting our witch world. One vivid memory I have is of a letter that I wrote in red ink, describing what I wanted to do to some boy in my class who had offended me. I drew a picture of him cut in half and reassembled the wrong way, with his legs and torso atop his head and chest. I was definitely not a nice witch!
The thing I craved in my little girl world was power. Power to hurt those who I felt had hurt me. Power to lord it over those to whom I felt invisible.
The idea of a young person with infinite power scares me. A person whose sense of the other is not developed, whose moral sense is similarly undeveloped, is truly unnerving.
Maybe that is partly why I chose to cast my Macbeth witches, the 'weird sisters', as adolescent girls. I've seen some pretty scary girl behavior in myself, and I've seen it in my daughter's world when she was in middle school & beyond. It can be vicious; though of course, not always!
Another thought is that, if you were an evil spirit, or some sort of dark energy, able to take on human form, why would you take on the diminished form of an old person? Wouldn't you want a young, beautiful, strong body? I sure would!
We know that women were labeled witches in old times as a means to persecute them. We know that they were not witches at all, but simply women with knowledge of herbs, or healing, or unusual or different sorts of women.
These are not the sort of witches you will find in our Macbeth. Our weird sisters are channelers of the natural destructive energy in the world; they are not human, but supernatural. Let the fun begin!