If your end goal is just to remove the Activation Lock/Find my Device from the iPhone so it can be used by someone else, you can contact Apple regarding Activation Lock removal, assuming this relative was the original owner of the device and can still locate original proof of purchase. This can be done here. https://al-support.apple.com/#/additiona...
If the end goal also includes regaining access to the Apple ID, your only option is to walk through the password reset process, indicating where necessary you don't have access to whatever backup contact method and eventually you should get to a point where it will offer to initiate an account recovery. Keep in mind that attempting to recover multiple times after this request has been submitted can delay or complicate the process. It usually will take a few days for this to be done since I believe this information is manually reviewed before a password reset directions are sent.
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