Question about writing about someone's life

I'm posting this question everywhere I can think of so I'm sorry if you've read it somewhere else. Maybe someone here can help me.

I know if I'm writing a real life article or book about real people, I would have to have signed statements from the people in the book, but my question is-- if one of the people I was writing about, was dead, how could I use them in my book without having a statement from them? Obviously, I couldn't get a signed statement from them. I'm talking about writing a book about a family member's life, but that family member is dead. I appreciate any help with this matter,
and Thanks.

Mary J. Dressel
http://www.maryjdresselbooks.com
The Enchantment Series
Visions of Enchantment and
Enchantment's Embrace