Is AUTHOR wrong in the post ? POST : Throwaway obviously.

Recently my sister started smoking hash/weed. She ended up going to a couple of my close friends asking if they could hook her up with a gram. 

My friends knew who she was so they contacted me and asked if they should or not. I told them no and that was the end of that. 

Then she asked them again... and again... in which case i WARNED her that i would tell our parents if she didnt stop because... well she is 13, and i firmly believe that anyone under the age of 16 shouldn't even consider touching ANY drug. 

Then she asked me and my friend again, this time i decided to scam her for $15 dollars since i really didn't want to tell our parents (I'm 17 so i didn't really want to admit i smoked). 
She is getting her money back because i don't like scamming a family member (or anyone for that matter). But thats besides the point.

She ended up finding another friend of mine selling xanax and weed, he told me he met a customer that looked an awful lot like me. I asked if her name was (her name) and he said yes. 

That was my last straw, i decided 2 hours ago to call up my mom, tell her everything that had happened and that i want her to set her straight without involving police ect. Ofc, somehow she wasnt surprised about me smoking but was really upset that my 13 year old sister was. 

She ends up giving me a 40min talk about harm reduction and alot of what she said i already knew but made alot of sense. 

She calls my little sister, and my sister ends up calling my mom "unsupportive" and "a horrible mother". My mom started crying and was almost late to her job.

After that, my sister calls me an asshole, and threatens to snitch on me to the police. At this point i don't care but i feel like i may have made the wrong decision.

AITA?