Bonjour
asking your sister's friends name
Comment vous appelez-vous ? (if you're asking your sister and if they're more than one)
Comment t'appelles-tu ? (same as above if ther's only one friend)
Comment s'appellent les amis de ta sœur ? ====> (what are the name of your sister's friend ?)
I wasn't sure of what you were asking for, it should be the last one ....