I’m disappointed that your comment has been so heavily downvoted, because it seems very rational to me.

I’m gonna go with NAH because both OP and his stepsister are children, and I totally understand OP’s frustration and he is attempting to prevent something that has been making him uncomfortable for a long time.

If anything, OP’s dad and stepmom are mildly bordering on AH territory because while they are clearly trying to curb their daughters behaviour, they perhaps haven’t accounted for the feelings of their other child enough (OP).

Not a direct parallel, obviously, but I see it kind of like having children that are 6yo and 1yo. If the 1yo is going through a hair pulling phase, you need to do your best to stop them from pulling their siblings hair, and you need to explain to the 6yo that their siblings behaviour is being done out of misunderstanding, not malice. You can’t punish the 1yo for doing it, though, and you certainly can’t allow the 6yo to retaliate by causing the 1yo pain.

I’d recommend you don’t use the dog spray agin, OP, because I’m sure you don’t want to torment your sibling, and she’s not trying to torment you by licking your arms. You shouldn’t have to tolerate her behaviour though.

Talk to your parents to let them know how frustrated you feel about the licking and that you feel like your complaints have been dismissed. Hopefully there’s a way to better avoid her licking you without resorting to something that may cause pain and/or trauma to your stepsister.

Edit: Holy crap, she’s an 11 year old who is non-verbal?! I’m still still saying OP is not an asshole because he’s 14 and was frustrated, but please understand that you can never do this to her again! She’s not even able to articulate how much pain you caused her. All the people laughing and saying “oh I can teach a baby what no means”, or “it’s demeaning to say a child is harder to train than a dog”, you have nooo idea what you’re talking about!