I was never one to listen to my mother, and she told me I could never have a snake. I got a snake anyways, but my mom was by no means terrified of snakes. I knew my mom wouldn't do anything once it was already here because that's the way she is. I'm not necessarily condoning going against your parents, but sometimes it can be ok to.
If educating her doesn't work and she is *truly* phobic of snakes, you need to respect that.