how to reply to oneesan ga naninn imasuka?

One possible way to reply to "oneesan ga naninani imasuka?" is:

"Oneesan wa naninani imasu."
(お姉さんは〇〇〇〇〇〇〇〇〇〇〇〇〇います。)
Translation: "My older sister has (insert information about what she has)."

Please note that the response will vary depending on what specific thing the older sister has.