feel it out like WhiteRav3n said. it sounds like your mother would understand if you are given the chance to explain. sit her down and calmly explain yourself. ask her if you could explain in full [so she doesn't interrupt and everyone starts yelling] get a book explaining wicca [Wicca for Life by Raymond Buckland is my favourite] and ask her to read it. explain how there is no devil in the craft, magic is not evil, the intent of the caster will predict if magic is good or evil. explain the Lord and Lady, how your faith is earth based, and try to use examples from Christianity. Yule's parallel is Christmas, explain that one, tell her how prayer and chants/spells are very much the same thing, asking a greater beings for help.
if you're under 18 it's mandatory you tell them [when the time is right] but if you're older then you don't have to ever tell them, but what it sounds like you're close with your mother, and she'll understand. trust me, it's a huge weight lifted off you when someone you were afraid to tell not only knows, but understands and supports you. i was always worried to tell my grandparents, one day i found out my mom told them and they were fine with it, i was so happy i cried [which i never cry] it felt so wonderful.