I got my sisters old phone because mine broke but it has activation lock and she doesnt remember the account details to remove the lock.

'Activation Lock for iPhone, iPad, and iPod touch' is a security feature that's designed to prevent anyone else from using an iPhone, iPad, or iPod touch, if it's ever lost or stolen. To learn more click here --> http://support.apple.com/HT201365

For an individual to remove an activation lock absolutely requires the information for the account that applied the lock. Click on this link to learn 'How to remove Activation Lock' --> https://support.apple.com/HT201441

Here are additional resources to be able to provide assistance in doing either of these:

Help with a lost Apple ID or password:

• "If you forgot your Apple ID" [this is likely an email address you are using or used in the past] click here --> https://support.apple.com/HT201354

• "If you forgot your Apple ID password" click here --> https://support.apple.com/HT201487

If necessary and you are willing to wait, Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).