I have direct experience with this, when my sister died 11 years ago. We never even worried about her email UNTIL my mom was getting spam email from her deceased daughter. Seems a spammer got hold of her inactive account. Same situation we did not know her password. After many, MANY emails and a few calls I finally got in touch with Yahoo legal. All I wanted them to do was change her password to prevent the spammer from accessing her account. They wanted probate documents (interestingly I work for the Philadelphia Register of Wills). Unfortunately Yahoo was the only place that wanted this. She died with a surviving spouse, my brother in law. I was NOT going to make him go through probating the estate for just 1 thing. What I did instead was to filter my mom's email box so she would no longer get these upsetting emails.
I also give you my most sincere condolences.