Encrypt the file with a secure password or key, maybe using AESCrypt. Email the encrypted file to the relevant parties. Put the password to the file in your will (keep it under appropriate trusted guard, to be released only on your death). As long as the will and the encrypted file are kept apart until after your death, the file will remain secure until then. You can also modify the encrypted file as things change, encrypt with the same password, and resend the file.
There's still the possibility that their computer is compromised after you die and they decrypt the file. They could reduce this risk by opening it only on a known-secure system (e.g. an Ubuntu LiveCD boot), if it really matters. In any case, this greatly reduces the security exposure by not have this file sitting around for years for anyone to read.