Sister problems

My step sister is moody as fuck, all the time. We recently started hanging out, we hadn't seen each other in years (14 yrs). We mostly hangout when she needs a ride or no one else is there for her. But if I can't hangout or do what she wants she flips a switch. She has used her daughter to guilt trip me saying "she misses you, come over". It will be a work day for me and she says come over, I get off at 10 but work late most nights (11:30/12). She recently blocked a fb status from my view saying how she pays for everything when we hang out. I make a lot more money than her, but when we do hangout she scams places fo food or coffee. I can only think of two times she has actually paid for something. She took me out for a bday dinner and then threw it in my face. We also went to the bar, she bought our group shots (4shots=$26) I didn't get a shot, I got one drink she paid for, $7. This week I paid my rent and literally had $5.14 in my account, she went to eat and I had to over draw in my bank account just so I could eat. She doesn't have bills to pay so when she gets a pay check she blows it. I pay bills with my checks. And when I tell her that I'm broke she seems to hear "I have a million dollars" it's my god damn birthday and I had to cash in my piggy bank for 8 dollars to feed me and my fiance. She complained about buying a birthday cake for her other sister but turned around and bought a $40 sweatshirt for the new guy that she's sleeping with. Ahhh, rant over. My fiance says to cut my ties with her, what do I do?