My bf and I have been together for 6 months and I recently met his little sister, she’s 21 and she’s so sarcastic. I’m not one to be fake and when I was talking to her I felt like I couldn’t even have a conversation with her. She’s very immature and she makes sarcastic comments to me when my bf isn’t around. This has never happened to me before, how do I handle this?

His older sister we get along so well