When a model or property owner is deceased, all next of kin need to sign the model or property release. Next of kin is generally considered to be, in order: a spouse, a person’s children, a person’s grandchildren, etc.
For example, if the model is your deceased father and he was not married, then you and each of your living siblings (your father’s children) would need to sign the release to permit his likeness to be used.
In the case of works of art, being left an item in a will does not mean that copyright passes to you and that you can sign a release. It will still be the next of kin of the artist who needs to sign.
Yes, we know it’s a pain.