My mom and sister are always bickering! What should I do?

I am getting really fed up. There is not a day when my sister and mom aren't fighting. My sister is always calling me up and telling me that my mom is getting on her last nerves. My mom also calls me up to tell me that my sister is being a pain and how she doesn't even want to talk to her anymore. They have been like this for quite a while now and I don't know how to help strengthen their relationship, if at all possible. I want to try and talk to them both, but they are both very stubborn and hard-headed. What should I do? I love them both and I really want them to just work things out.