Each Apple device is linked/associated with an Apple ID of the person who did the setup.
Your sister has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch -> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Then she has to remove the iPad from her account, see this Apple article:
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to find where you're signed in - Apple Support
Then you can sign in with your own Apple ID.