heh, I have the exact same situation with my little brother. he's always in my room and EVERY time I take some weed he keeps begging and begging like I owe it to him, I swear the kid smokes at the very least 1/3 of my stash.

anyway, I just tell him I can't afford to use so much, seems to work best. I mean seriously, if she thinks you're a bitch for not allowing her to smoke all your stuff and never share with you, she's pretty damned self centred.