Holy shit, you sound like you have my exact same relationship with my M.I.L.
My wife's sister sprung my set up and told every single god dam one of my in laws. The only reason she found out was because she feels she has the right to snoop around other peoples houses and search through cupboards and shit.
Anyway I banned my wife's sister from the house but now that they all know it's like I am the worst person in the world (I always was anyway, but that is a whole other story).
What makes me so angry though, is I don't drink, I don't smoke tobacco, I have a respectable well paid job, I 100% support their daughter (after they kicked her out for no reason), and I actually only smoke weed 2-3 times a week. Her M.I.L drinks, is addicted to nicotine, has a shit house job, and is generally a god dam moron. Yet I am the druggy because of a bi weekly relaxation hit.
I really hate being judged, my wife hates drugs but she always listens to my explanations of why I take what I take. I only take stuff that in my opinion helps me expand my mind and helps me look at life from a different point of view (weed, opium, DMT).
Sorry I seem to be ranting but your post brought back a lot of anger I have with my in laws. My opinion, ban her from the house until she apologises, also explain to her why you smoke weed.
-EDIT- Whoops just realised this thread is a year old LOL.