Quotes: My Sister Is Off-Limits!

"What a girl! I think she likes you. Word to the wise, though - she don't put out. Which is convenient, 'cause if she did, I'd have to kill you."
Patrick "Packie" McReary, Grand Theft Auto IV

"She's Ron's sister, Harry told himself firmly. Ron's sister. She's out of bounds. He would not risk his friendship with Ron for anything."
Harry Potter, who, of course, ends up married to her anyway.

"You wanna fuck my sister?"
"You wanna fuck my sister!?"
Stephen Lynch, The Best Friends Song

If your sister hangs around your friends, eventually one of them is going to hook up with her.
From a Spike TV commercial describing lessons learned from Star Wars

Private Joker: I wanna put my tubesteak into your sister. What'll you take in trade?
Trunks: "Goten's not your type."
Bra: "So what is my type? Since you seem to know what I prefer."
Trunks: "Someone who isn't Goten."
Dragonball fanfic A Lesson In Humility

(401:) Dude. My sister is off limits. Touch her again and I'll rip off your dick and force feed it to you.
(301:) I accept this challenge.
Texts From Last Night

(336:) If I die tonight, I want you to know that your sister is awesome in bed.

(813:) I'm fucking your sister right now.
(1-813:) You motherfucker.
(813:) She's next.
TFLN likes this trope. This isn't even close to all of them

Atticus: (expression souring) Bastion... Misawa. ...Hm. He's got it bad, doesn't he! He's got a crush on Alexis!
Zane: Does he?
Atticus: Absolutely! Are you blind?! ... As her older brother, I'll never allow it.
Can we not talk about my sister in relation to A: massive wide-ons and B: Steven's erect penis.
—- Oliver, The World's End

Ross Geller when he sees Chandler and Monica making out in the apartment, Friends

Bloodedge: (Sees Nine holding two fireballs.) "W-Whoa whoa whoa, wait a second! Hey! Just calm down for now, yeah? Tell me what I did!"
Nine: "What you..? Why do I need a reason to vaporize you right here?"
Bloodedge: "Wha...? I don't know what you're saying—!"
Nine: "Let me help you remember by beating it back into you! No one fools around with my kid sister! When I'm done with you, your limbs may no longer be attached to your body!"
Nine when she sees Bloodedge and Celica in the forest, Blazblue