"I don't have a sister."
Translation:Εγώ δεν έχω αδελφή.
Negative sentences don't have an indefinite article. In a positive sentence you would say "έχω μια αδερφή=I have a sister" and not έχω μία αδερφή=I have one sister" unless you wanted to say that she is one and only. The "έχω αδερφή" positively, without an article, would be used of someone told you -"you don't have a sister!=δεν έχεις αδερφή" -"I do have a sister!=έχω αδερφή!"
I also got this wrong. Is it always the case that one doesn't use a negative indefinite article? Should one then exclude it when one encounters it in the exercises? Is there a place where one can find out these grammatical Rules? This came as a complete surprise to me >:((