Before giving the iPhone to your sister what was the status was it locked with passcode or you disabled the passcode see this https://support.apple.com/en-in/HT204060
and then you signed out your Apple ID signed in from an iPhone https://support.apple.com/en-in/HT208242
And she had already an Apple ID and signed with her own created Apple ID in the iPhone https://support.apple.com/en-in/HT204053
Now she has forgotten her Apple ID she can reset the password https://support.apple.com/en-us/HT201487
The question you asked can she reset her Apple ID password with your existing Apple ID and password - the answer is no
Because Apple ID password is always reset for an individual account for an administrator and not using the other person Apple ID and password ( the accounts credentials can't be utilised ) , they are separate and confined to individual accounts .