dude don't get all pissy. just keep it on the dl if its causing you problems. trust me, you don't want to fuck up relationships with your parents. consider that she does this because she cares about you. yea you might not agree with her, but at least she wants to be a part of your life and she thinks weed is the only thing you are concerned about. my mom was literally the same way in that she would tell relatives even if she caught me drinking. i came to the realization that she really just loves me and wants what she believes is best. don't butt heads w/ her and just keep your smoking to a reasonable level. weed should enhance your life, not be your life.
good luck w/ the situation, tho.