How can I bypass a iPhone passcode of a deceased family member?

Without the passcode, you will not be able to get access to any data that exists only on the phone. If you enter an incorrect passcode too many times, I believe the phone will enter a state where you must reset and wipe it in order to get back in.

As far as I know, Apple cannot bypass the passcode, either.

Apple may be able to help an immediate family member – or the executor of your cousin's estate – to gain access to your cousin's Apple Account. If your cousin has been synchronizing photos to iCloud Photos, or doing backups that included them, it may be possible to recover them that way.

See: How to request access to a deceased family member’s Apple Account - Apple Support

You may want to pass along this link to your cousin's family and/or the executor of his estate. They're the ones who are most likely to be able to meet Apple's security requirements.

Sorry for your loss.

How to request access to a deceased family member’s Apple Account – Apple Support (UK)