Well, it is actually very easy to do this on your own. You can put the secret passphrases and/or Private keys in a time capsule and bury or hide it in a place that only your family members will be able to find it.

Then you leave clues to your treasure with someone you trust or a lawyer (break it up in 2 or 3 templates that can only work if all parts fit together) Once you are gone.... they can get the pieces together and solve the quest to find your coins. (The clues must be something that more than 3 people would be able to know about you)

A lawyer only has one part of the template and the other people only their pieces... so they cannot do anything without the full treasure map.