I use Nxt, and my Nxt priv. key is on the desk next to me.
A solution to this problem that is safer than my piece of paper is Multisig; you could require both your family, a lawyer, etc. to approve a transaction in order to get have the support them.
the family would have to be educated about what bitcoin is first in case either the lawyer dies or he scams the bitcoins which maybe worth several million doillars in say ,a decade from now .........
you need to make your family aware of btc,wallets and passphrases imo b4 its too late
a paper copy and instructions in a sealed envelope in a sd box wouldnt hurt either in case they forget or lose any part of the info required to "spend" the btc ........