Have you sat down and talked with her about this? Do you know what her exact objections are?
My best advice, be as calm and adult like as possible, and don't push her too hard all at once. Maybe show her how much you know about taking care of them and how harmless they are even at full size. Cats and dogs kill/hurt far more people each year than snakes do.
But you need to understand that at the end of the day she has the final say. You might just have to wait until you are out on your own. Try to stay positive.