Translation:They are sisters.
I can't understand why かのじょうたちはきょうだいです isn't right!? The word きょうだい could be used as for brothers and for sisters as well. Both answers should be right. It's like a parent. Why could not i say about my mother or father... he or she is my parent. There is no restrictions by gendre.
There is a word for "brothers", which (thanks to patriarchy) is used as a standard "siblings" for all groupings that include at least one male, and then there's a separate word for "sisters". Since this sentence is only about かのじょたち (mind the short '~jo') only the latter is correct.