Law InfoNet

My sister couldn't get an apartment in her name so she asked me to lease it for her. I did and she got evicted. I just got married and my wife and I were in the process of purchasing another home, so we pulled my credit report and noticed a balance of approx. $4000. We went to my sister several times and she said she would have to make payments, but every time we asked for a payment she always had a sob story. So my wife called her and explained to her that I didn't want to take her to court. She got mad and said she would mail us a payment, but she was changing her number and she wasn't going to talk to us again. Now we can't purchase another home until the debt is paid. What do I need to do?