Hello!
Answer:
Comment vous appelez-vous ?
Step-by-step explanation:
→ They are "two sisters" so it's at "pluriel".
Comment vous appelez-vous ?
→ So we use the "vouvoiement" them since there are two of them.
→ If there was only one person we use the "tutoiement".
→ So the sentence is:
Comment vous appelez-vous ?