Uhm, I don't understand why everybody says that it doesn't make sense. For me, as a Spanish speaker, the sentence "Ella añade que no tiene hermanas" sounds completely normal. For example, she is speaking with a guy, and he's asking some personal questions and "ella añade que no tiene hermanas".
That makes sense, but this seems like it should be more like a secondary sentence, as if she was already talking about how many brothers she has, and THEN add that she has no sisters. Just seems weird to blatantly say "she adds that she has no sisters" without any background info.
This makes perfect sense. In case you don't get it, it could be describing the end of this situation without directly quoting:
Person A: Are you sure you don't want any? Here, take some to your sisters...
Person B: No thank you... ...and I don't have any sisters. This sentence just means that she already said something, and then she supplemented what she said with more information: I don't have any sisters.