Issues with ex housemate

Ok my sister's stepdaughter and I lived somewhere together a few years ago. Our apartment at the time had gotten damages from her friends kids, and ourselves. When we moved out, I got billed for the damages since the lease was in my name and she was on it as the occupant. I kept asking her to pay for half of the debt for over a year before she got my sister involved and jumping my ass. Then finally last year she started paying after my boyfriend paid $600 and I took out a personal loan and paid the rest. She doesn't pay us every single month like we verberly agreed to. I am going to write up a document that we all can negotiate on payment arrangements and agree on to cover all of our asses. She won't reply to any of my test messages, she is 34 fucking years old, she gets my sister involved and my sister calls me a bitch and a bully. Things with my sister's stepdaughter and I was really good until she faked a pregnancy for attention and she's just a cry baby. The way she ended the pregnancy was she claimed that she fell down the stairs and the baby died full term and she came back home that day, getting all of my family to grieve for a unborn baby and my cousin at the same time. When she was having issues with her car I suggested that she paid us $20 a month until she got a new car and she can start paying us $40 a month until her half of our debt is paid for. She got her new car a few weeks ago so my boyfriend and I want her to start paying us a little more than $20 every month She lives someplace that goes by your income so I know that she can afford to pay us $40 a month and she doesn't have any children or pets to support. Why can't she just fucking grow up, stop getting my sister involved and communicate with me like an adult?