How to reset an iPhone?

You are probably encountering Activation Lock which is a theft prevention feature. Without the Apple ID and Password of the original user,you will not be able to use the phone. Since it was a relative, see this link for a possible solution...

How to request access to a deceased family member's Apple account - Apple Support

Hello Suffolk1

My condolences.

If your brother's iPhone is locked, with iPhone Passcode, then it needs to be reset. However, you should be aware that if the phone has been signed in with an Apple ID and Find My has been activated, you will be asked to enter the password for your brother's Apple ID.

So even if you reset the device and the passcodes are removed, Apple ID will ask for the password for his Apple ID.

See this support article, it may help reset the device:

If you forgot your iPhone passcode - Apple Support

See also this:

How to request access to a deceased family member's Apple account - Apple Support