Oh, well that's it then. Take it to your parents, with a mature conversation (no raising your voice, whining, crying, begging, etc..etc.) and without your sister there. Tell them the situation, and then ask for their suggestions of what you can do to end it.
"Hi mom, dad, can I talk to you for a minute. Listen, I'm a little older now and I was expecting some privacy to come with that. But (little sister) has a problem with barging in my room constantly, and bothering me while I'm trying to concentrate on things like reading, writing, or my homework. And that become a problem because I'll start fighting, yelling, etc.. with her which usually makes you mad. So, is there any way that you guys can help me get some peace during the day? Maybe if you spent more time with (lil sis) or allowed me to lock my door, it would really help me out." etc..etc..etc....If they want to bullshit around with answers
"Well she's your sister/she just wants to be like you/she loves you is all/you should be spending more time with her...just say,
"Well I could/would if I had more time to myself, but right now, the only thing I can do with my freetime is yell and scream at her to get out." etc..etc..Edit to your circumstances.

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