I have a little sister myself and i couldn't even imagine behaving the same way to her as you are to yours, that's family, your flesh and blood, you should support her as much as possible for as long as necessary.
She shouldn't be working whilst going to school, that's a mistake a lot of students make in college, especially ones that study intensive courses. The time she spends working to partially provide rent (rent that you've already admitted you cover comfortably), is time she could be using to sharpen her grades so that she can pass with the best results. Too many responsibilities apart from studying will take away from her study time significantly, which may consequently lead to her failing, or do you not care about that slim?
This is how deep family rifts are created, if she struggles and fails partly because of your callous attitude, she will blame you, she will basically hate you for life and so will her children (I've seen this happen), things can get very ugly that way.
That's your sister, support her unconditionally if you can, if you show her nothing but love, that's what you will get back; she will probably work harder in college knowing the sacrifice your making for her and she will remember your kindness for the rest of her life. Like wise; show her unnecessary callousness and harshness, then that's exactly what you will get back.