hi, my apple id suddenly changed to my sisters email without us changing it. then when i edit the email back to my original email in apple id, it can no longer identify my email

I know of no process where it could do it on its own. There are situations where you might see another ID such as if an app was purchased with a different ID, but you don't outline specifics as to where you are seeing this ID and it is hard to tell if it is one of those.

If the email address was truly changed on your account, you cannot change it back to an email address used before. Apple does not have a statement about email address re-use in its documentation that I can find but here is an April 2021 post by ASC contributor IdrisSeabright that was recommended by the official Apple team -

"Once you've used an email address as an Apple ID, it can't be used again for an Apple ID, even if the first Apple ID has been deleted."

https://discussions.apple.com/thread/252629414?answerId=255095641022#255095641022

Your other concern in this situation is how/why the email address could be changed:

Click on this link "If you think your Apple ID has been compromised" --> https://support.apple.com/HT204145

'Checklist: If you want to see if anyone else has access to your device or accounts' click here --> https://support.apple.com/HT212021

Security and your Apple ID - https://support.apple.com/HT201303

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Better for directly reporting an issue.