A friend's sister is a very special status that needs to be addressed in a cordial and respectful way.
First of all, my friend's sister is a woman, so we should use a respectful way of addressing women. This can be "lady", "wife" or "miss", depending on their age and marital status. If she is much older than us, we can call her "Auntie" or "Auntie", and if she is much younger than us, we can call her "Little Sister".
Secondly, the friend's sister is the friend's relative, so we can also address her in a cordial way. In Chinese culture, we usually call a friend's sister "sister", which is a very affectionate way of addressing her.
Finally, we should choose the most appropriate way to address ourselves according to the relationship between ourselves and our friend's sister. If we're good friends, we can use more affectionate ways to address us, such as "sister," "sister," or "little sister." If we are strangers, we can use more formal ways of addressing us, such as "Ms.", "Mrs.", or "Miss."
Whichever way we choose to be called, it should be based on respect and kindness, showing that we value and care about her.