The easiest solution to this is not to get involved in any matter with these sisters. They may honestly not know if they're doing any wrong or not. If they try to speak to you casually and make conversation, tell them you cannot because it is not permissible Islamically to have a casual relationship of any kind and that should be sufficient. Also, you should try not to be in an environment where ghayr mahram muslimahs are going to interact with you.