That already exists. A will is most often in a sealed envelope, and you just need to add to it the keys to your wallet. Lawyers have been handling those kinds of things for decades.
The problem is, do your family know bitcoin? I guess even with last wills, you're family will not be able to know those ifthey didn't know what bitcoin is. So let's say they know what bitcoin is, then why do we need to pass it to the lawyer if we can pass it to our family directly. I mean yes you don't know when you will be dying but I guess you can say something to your family that can give them a clue about it right?