In English, you can use either "sister" or "sisters", but it's more natural - or at least more common - to say "sisters", so that is accepted as a translation. But the fact that you can interchange them freely in English doesn't mean you can do the same thing in German.
Duolingo is indeed wrong here (following a number of posters below), because the translator is unfamiliar with colloquial English, and thinks that the student has incorrectly noticed that the sisters are plural rather than singular. But it is an "understandable" teacher error at an elementary level.
Well, let me see if I can help you. When you use the auxiliar "Does", then you have to use the regular form of the verb. In this case, "Have". You can say "She has" or "She hasn't", but if you use the aux, then would be like this "She does have", "She doesn't have". The same applies to other verbs. Example: "He eats", or "He does eat". In the last, note that the "s" is not used. Hope it helps you.