Hey I feel your pain. It's too bad your GF's older sibbling isn't a brother. I'll explain...
My wife has 3 older brothers. They are, for lack of a better word, rednecks. That's not a negative statement, however, they don't handle situations that well when it comes to emotions. I had a hell of a time when I started dating Sarah, my wife. They hated me. They would litterally call Sarah at 9:00pm when I was visiting and tell her to tell me to leave because it was getting late.
By the time I had been going out with Sarah for 6 months, I was getting very tired of the shit they were pulling. The last straw was when I was over at Sarah's mom's place, when her mom was gone, past 9:00pm. Her brother came over, came into the house, and picked me up by the shirt and started to throw me out. This pissed me off beyond anything I could handle.
Now, keep in mind, her brother is huge, very stalky, mean as hell and thick. I took him out, and I to this day do not know how I did it. Next thing I know, he's on the floor, and I'm beating the ever loving shit out of him with Sarah on my back trying to pull me off going "THAT'S ENOUGH!"
After that they didn't have a problem with me.
Stand up for yourself, but do it the right way
You obviously can't ... or shoudn't... hit a girl. Just come right out, get in her face, and say "Hey, I love your sister, and I will not let some superficial zit such as yourself come between her and I, so unless you want to get popped, keep your infected self out of my business."