In my head? The same place I store the passphrase to my password manager.

With no backup? What if you suffer a massive head trauma (or an unfortunate death), no way to recover your millions of dollars for friends and relatives? I'm guessing people put this stuff in their wills these days, how secure is a will?

I have an encrypted file with online banking passwords, account numbers, etc for my wife.

It's on a USB stick in our fire safe, and also in our safe deposit box along with a passphrase hint that my wife or daughter would understand, but is not obvious to an outsider.