HELP! How can I get my best friend to stop talking about her sister?

My best friend, Melanie has a younger sister - Jessica. Jessica is 4. She is really spoilt and everyone in the family loves her and lets her do whatever she wants, she gets away with everything. Their family live in Germany now, and we live in England. Our parents are friends, so 2-3 times a year their whole family comes to England for a few days and stay at our house. I really dont like Jess because she is so spoilt and stroppy and expects you to do everything for her and treats me like I owe her something. Now, Mel has been at my house for nearly 2 weeks. She came for a whole month. She talks about her sister a lot and it is getting a bit annoying. I have enough of seeing her when she comes, I dont need to be told about her when she isnt here. How can I ask my best friend to stop talking about her sister? She always says stuff like 'When we went to __, Jessie did this. It was so cute!' 'Mum bought Jessie a new dress. Its so cute!' 'Did you know she has natural highlights? Jessie is so pretty' I really like spending time with Mel, and I appreciate that its nice that we have a whole month together, but I just wish she would stop talking about her spoilt, stroppy sister. I cant say to her face that I hate Jess because that will be really rude and asking Mel to stop talking about Jess indicates that I dont like her. I have tried hinting like, each time she brings up Jess, I listen to half of the story, then say 'Cool. So, what do you want to do tomorrow?' or something like that and change the subject, but she still talks about Jess and finishes the story after she answers my question. What should I do?