Tip - Maid of Honor: Must Married Bridesmaid Be Matron of Honor?

I have a maid of honor (my sister) and six bridesmaids. I want my sister to be the maid of honor, but one of my bridesmaids came up to me and said that she was surprised that I didn't ask her to be my matron of honor. She told me that a matron of honor was a member of the wedding party that is married. I have learned that it is only *one* person: maid of honor if you're single, matron of honor if you're married. What should I do? I don't want any hurt feelings, but I only want one wedding honor attendant, and that's my sister.