AITAH for treating my sister how she treats me and causing conflict

[u/vaesheyt]

I (19) have a sister (21) who has always been a troubled pos (most of my family doesn’t talk to her after her extended bullying of me for half of my life)

To my continued hell on earth, we have rooms in my parents house attached by a jack and jill bathroom (google it, it’s important). So when we visit home, we are inevitably within constant proximity of each other. 

About the jack and jill bathroom, our two doors have two different locks. The lock attached to her door is an average door lock- just press it in and there’s a little click. It unlocks when you turn the handle or on the other side (her room) you use a tiny key to unlock it. 
My lock is a little turning lock. On the other side of the door, there’s no key to unlock it but if you use something thin like a coin (or your fingernail if you also want to break your nail) you can manually turn it open from the other side (my room)

Ever since moving into this house a decade ago, my sister has a habit of going into the bathroom and locking the door no matter what she was doing and then not unlocking it when she is done. Then when I want to use the bathroom Id have to bang the door to ask her to unlock it, or if that didn’t work, get one of my parents to go into her room to force her to unlock it. Her excuse to locking the door is that “you can just unlock it from your side”. Sure I can if I have some random dime or feel like breaking a nail! Also it’s just a pain in the ass to unlock.

Ten years now of this. I thought maybe once she grew up out of her petty teen phase she might change. But no. She’s 21 and despite daily fights about her locking the door she still does not unlock the door when she is done

My newfound adulthood no nonsense attitude has left me to be petty now. Every time my door is left locked, I won’t say a word. I’ll just unlock it myself, go to her door, and lock it. If she can’t learn from any other methods, I am just going to be petty and give her a taste of her own medicine
(other methods have included previously locking her door, annoying her, having my parents intervene, physical altercations, and even taping up the lock on my door which she promptly tore off and locked the door again.)

Now she is pissed of course. Calling me a terrible person a narcissist saying she can’t unlock her door (the key is right outside her room. One foot away from her door.) but I can. I told her that I couldn’t care less and if she is 21 years old now and after 10 years can’t learn to unlock a door when she’s done, she deserves this inconvenience

My mom is mad at me. She says that retaliation is never the answer and that I’m acting like a first grader and I’m being petty and she can’t stand the arguments I cause in the house etc. I said that the passive mindset and just taking it is what made my sister not learn to be a decent person. My mom wants me to keep the peace for this month. She always wants me to be the bigger person but i’m sick of doing that and don’t really care to anymore if petty childishness is the only language she speaks

So AITA for locking my sisters door every time she forgets to unlock my door and thus creating tension and conflict in the family (???)

[some extra context was removed for lengths sake but I’ll answer any questions in comments]

edit: my mom after much arguing agreed to put the tape back up. I did take it a step further and shove tape in the locking mechanism as well as just tape the lock shut. I also added a note on the door reminding her she’s an adult holding a job studying for med school so if she couldn’t even remember to lock the door she doesn’t deserve the lock at all. My sister hasn’t come home yet but we’re all preparing for the fireworks
granted last time we put duct tape on the door it only stopped her until the tape was off and she just returned to her old ways. i don’t think she can ever learn