Well there's the traditional way of having a will but it's understandable that a third-party is involved if you have that since you need a lawyer to legalize transferring everything you own to your heir. If you want it to be more private there's paperwallet, your btc will be safe if you can physically hide the paper well and only your heir will be able to get it. It's a smart and subtle way of hiding your possession well.
You don't actually need a lawyer for that because as far as I know, there's common law and banking law that governs these things. Under common law, you don't have to "report" any cash found at home.
Imagine that you live with your mother and she dies. You are the only child and your father is dead or your parents are divorced. Whatever movables were there at home are automatically yours. If she had cash in a safe and you know the combination, it's yours. Nobody is going to ask you how much money, jewelry, or art pieces she had as long as there are no other heirs that can lay claim to these things.
If she had a bank account however, the bank will not grant you access unless you give them a court order.
With bitcoin there's no third party locking you out. If your mother left you private keys, seeds, or wallet passwords, the money is yours.