Since crypto enables us to have trustless relationships (and that IS a key feature of cryptocurrency) I think it's not a good idea to hand over your private key to some third party (lawyer). It is a solution but it is an old fashioned one and the security of that is doubdtfull.
There is a project that allows you to make a will and dispose of your crypto-currency assets. This is based on smart contracts. Smart contracts control the fulfillment of your will. So at the moment it's not a problem.
Do you have a link to that project?
As it sounds to be done in a decentralized fashion it could be interesting.