How do you tell your parents it's not your job to take care of them?

I'm a 22 yr old girl the oldest of 2 daughters. I moved to NY with my boyfriend about 2 yrs ago from NC, which is where my parents live. My sister just turned 17 and my parents are young (they're both 43). Well my sister is an honest to god disaster, she's on drugs, ually active and she has decided to quit school, my parents do not know how to set boundaries for her so they try to make her my problem. They call me every time there is a problem with her, they also ask me to help support her FINANCIALLY as well as them. My boyfriend and I are not poor by any means but I am in college and dealing with a lot of stressful situations of my own. My parent's make me feel guilty and obligated when I don't help them and they also get very angry with them. I don't know what to do about this I've had to see a therapist over this whole scenario I'm just wondering if anyone else out there has had to deal with deadbeat parents. Don't get me wrong I love them but they're LAZY!!!