Im a stepdad to SD17,Sd15 & Sd12 punched SD15 in the back and have a very close relationship with SD15, but recently its started to become weird and I'm not sure how to fix it.
She is known to be a problem child, she's extreemly loud, tells people of her opinions very openly, we laugh a lot, but also fight a lot because of this openess.
Recently she has been 'jokingly' hitting me and pinching me to the point where it has left bruises on my arms.
Everyone is a little shocked by it if she does it in front of relatives. I take 2 approaches, I tell her off quite harshly and also sit down and talk to her and ask her why shes doing this and explain its not right to act this way etc... but it seems to keep happening... she also does weird things like lick my hand....

Another thing that happened that I am not proud of was during an argument because she cant her younger sister a c*nt, she pinched my bicep quite hard and I reacted by giving her a sharp dig to her arm which made her cry... well because of our relationship, were ok about that now.. hopefully she wont bring it up in therapy in 15 years time!!
I find that I am constantly nagging at this girl just for her to act normal, not be mean, spiteful, rude, aggressive.....

I really dont know how to bring back 'balance' to this relationship...I realise it must sound very strange...

Please give me your opinions and advice, and be open, I wont be offended!