Am I being a bad sister?

My sister is extremely needy. We're both in our twenties (I'm three years older), and we live together, but she has almost no social life and hangs out with a friend or two just 2-3 times a month, whereas I spend time with either friends or my boyfriend about 4-5 times a week. When I come home, she leaches onto me, follows me around the house and talks to me about anything and everything for at least 15 minutes straight..sometimes an hour. During this time, I will talk with her too, while going through my mail, getting ready for bed, etc. She talks about a cat she saw walking across the street. A science documentary she watched about atoms. A cloud she saw that was shaped like a boat. Boring ass crap. But I still try to be nice and say, "oh wow! A cloud..how cool". During this time, my bf might text or call to say good night, or some other similar distraction. My sister's reaction? "oh, I am taking up your precious time. Sorreeeee." and stalks away, proceeding to ignore me for the next day. I know I'm being petty as hell, but I need to vent, and could use some advice as to how to not let this bug me. It would also be nice to know I really am being a bad sister. (background note: we were raised to be passive-aggressive as hell. Ugh. I've also given her over $12k by paying her credit card bills for the last few years because she was a full time student and I work full time. Enablement at it's finest, I know. But would one say she is "biting the hand that feeds her"?) Thanks in advance.