I have always just used first and last names when writing my nieces and never used the title Miss. They are 14 and 16, and my sister mentioned that the girls are now titled Ms. Is she correct and what is the plural form?
Dear Missed Miss,
You can title correspondence to your nieces as Ms. or Miss. The plural form for Ms. is Mses. or Mss.; the plural form for Miss is Misses.
Generally, Miss changes to Ms. when a young unmarried woman reaches the age of 18, but you can ask your nieces what title they prefer, Ms. or Miss.
Robert Hickey states that using Miss "is narrowing to very young girls. Young women as early as their teens may prefer Ms." (Hickey, 2009).
However, Nancy Tuckerman (The Amy Vanderbilt Complete Book of Etiquette) states that an envelope is addressed to a young girl with no title, just her name (i.e. Jane Smith) until she becomes a teenager, then she becomes Miss until age 21 where she can be called Ms. (Tuckerman, 1995).