Your sister needs to talk with others at her school or with adults who are helping with organization of the dance.
Dress codes for middle school dances vary from place to place and occasion to occasion. The most appropriate attire may be jeans and a nice top; it may be a pair of dress slacks and a top with lots of "bling" factor; it may be a dress or skirt. What she wears may depend upon her circle of friends and what they are wearing for the dance, whether or not there will be certain types of additional activities before or after the dance (eating out, roller-skating party, movie night at a friend's sleepover), and if she is dressing to impress anyone special who will also be at the dance.
Bless you for wanting to be supportive, but she needs to give you lots more information before you have any basis for giving advice.