My family member hates me?

My sister whom im really close to and her daughter has gotten into a heated arguement and the daughter didnt want to send her children to her mothers anymore and both of them were calling me for advice and I told them.I am not taking sides but I will listen to you both.They asked me to relay messages back and forth which I knew I shouldnt because I would get the blame for things said..But I did anyways..Know since the mother and daughter made up but denies saying the things she said because she got caught in a lie and wont fess up so she tells her mom that I (Me) am lieing and she never said any of them things...So the daughter calls me up wanting to start an arguement to make herself look good and I knew she was going to do thins and I was prepared 4 her to call..We got into it on the phone Bad and I called her a lieing biach and I will not speak to her again until she tells the truth and admits that she DID say all them things.She denies them and I know she is lieing.What should IDo