My friend cant stop talking about her sister?

My best friend Melanie lives in Germany. I live in England. We are both 15. She used to live in England but moved. Melanie has a 6 year old sister called Jessica. Once every few months Mel comes to stay at my house in England. When she comes she constantly talks about her sister and tells me all the 'updates' on what her sister is like now. For example, in the summer she came and kept saying 'Jess started gymnastics, she is amazing!' 'Did you know Jess has natural highlights? They are so pretty' 'If Jess was here she would love to do this!' and to be honest - I dont care. Sometimes I come to stay at her house and Jess treats me like she is superior. For example I will be talking to Mel, then Jess will come in and start whispering to Mel or say right in front of me 'Come on. Lets go there'. She interrupts our conversations and treats me with no respect. I dont want to hear about her and I want her to have respect. What should I do?