Ask you sister to give you her Apple ID password. If, for some reason, she can't give it to you, then eventually you will need to be able to answer her security adequately. If you think you will be unable to do so, then see:
Security and your Apple ID https://support.apple.com/en-us/HT201303
This document says “without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account.”
Thus, we can do nothing to help you if you won't be able to answer the security question acceptably, except to mention that Apple support might help you if you have an extenuating circumstance. From what I have read, I don't think you have one. Nevertheless, if you want to contact Apple support to discuss any extenuating circumstance you think you may have, see these support options:
Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362
Apple ID Support https://support.apple.com/apple-id
Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.