Yes, "Not unless" means the same as "No, unless." The sentence "No, unless she asks you" would be well-understood. "Not unless" is a shorter way to say "You should not, unless"; and the sentence "You should not, unless she asks you" would also be correct here. However when two people are speaking, it is not necessary to specify the other person as "you", so English speakers usually drop that word. Using a longer version would sound less natural to an English-speaking listener; but the sentence would still be understood. Another time English speakers drop the word "you" is in imperative sentences, commands, such as "Do not tease the cat."

Thank you for your comment^^

Synonymous expression: "only if."