That stinks. I'm 24 and had no real family to speak of until just last year, and everyone knows I like to smoke. I couldn't imagine hiding something from family your close to.
My mother once came out of the shower in a robe, and I handed her a bowl packed with tobacco from a smoked newport, and I told her it was clogged from resin so she had to hit it hard. She did, and she literally pissed on the floor. That shit was funny.
If it bothers you, you should just tell her. Nothing bad can happen from it, just don't smoke in front of her. Unless you guys don't get along already.