"You have a very interesting sister."
Translation:У вас очень интересная сестра.
My answer was marked incorrect "У тебя есть очень интересная сестра". I was tempted to report it but there may be some good reason why it is considered wrong. So if anyone knows why this may be, please let me know :)
According to my understanding, есть is not required here because the subject of the sentence is that she (the sister) is very interesting; her existence is already implied.
My understanding is that you'd only use есть if the existence of the sister is in question. Since the listener presumably knows they have a sister...