Cops Are Now Opening iPhones With Dead People's Fingerprints

No privacy for the dead... it's entirely legal for police to use your fingerprints to unlock your phone after you're dead, even if there might be some ethical quandaries to consider. Once a person is deceased, they no longer have a privacy interest in their dead body. That means they no longer have standing in court to assert privacy rights.

Relatives or other interested parties have little chance of stopping cops using fingerprints to access smartphones too. "Once you share information with someone, you lose control over how that information is protected and used. You cannot assert your privacy rights when your friend's phone is searched and the police see the messages that you sent to your friend. Same goes for sharing information with the deceased - after you released information to the deceased, you have lost control of privacy... (Forbes, March 22, 2018)
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Whether you are dead or alive, I recommend that you use a long passcode to secure your smartphone (do NOT rely on the fingerprint reader). The fingerprint reader can be bypassed by anyone who can force you to press your finger on the sensor thereby unlocking your phone and revealing your data.