Now, here is my solution. I can create a private key using 20 words. And I can make the last 5 words something only I know. I can email my family (and myself) the first 15 words. Then, I can email them the clues of my last 5 words. Of course, I have to do a damn good job that only the ones I fully trust know the answers. So, I guess my method is 3/4 paper - 1/4 brain wallet?
Apart from the risks from your proposal: why would you give your family access to your Bitcoins? If it's meant for the "hit by a bus scenario", there might be another option. I've been thinking about this for a while now, but haven't actually used it:
1. Print a normal paper wallet.
2. Sign a transaction to send funds from your cold storage to the normal paper wallet, but add a
Locktime so it's only valid from a block far far in the future. Say 1 million block count or even more (but not so far none of your family members will still be alive). Or while you're at it: create a few versions (1 million blocks, 1.5 million, 2 million) and print them on different sheets of paper.
3. Print the transaction and store it with the paper wallet.
4. If you're still alive a couple of months before the first transaction becomes valid: burn it, the next one becomes your new fail safe.
If your family ever needs it, all they have to do is wait a few years, broadcast the transaction, and the paper wallet becomes valuable.