Well, your attorney would probably better know than any of us.

However, from a strictly technical/mathematical perspective, these are extremely strong options:

A) Just Shamir Secret Sharing the document's password with the trustees and attorney and hope for the best. This would require at least two of them to be able to open the document.

B) Since you're still alive, after Shamir Secret Sharing the document's password with the trustees and attorney, it may be wise to also encrypt with your attorney's GPG public key or some passphrase known only to the attorney. That way initial access and any possible leak can only come from your attorney.

On your death, the attorney GPG decrypts the encrypted package to reveal the SSS document. Then either of your two daughters or a daughter and attorney can get the password to open the document.

C) Another alternative is to use two layers of GPG, one layer to multiparty encrypt with your daughters public keys and then another layer to single party encrypt with the lawyer's public key. That way the attorney again is the only single point of access. After the attorney has decrypted the document once, either daughter can access the document, i.e. the attorney can just give a first stage decrypted copy to each daughter and each can do the last stage decryption independently.