Contact the family members that you'd like to write about. Some may not be crazy about the idea while others will fight for attention in each chapter. Forewarn everyone.
Consider using an alias for a family member who does not want to be featured in your book. You still have every right to write whatever you want, as long as you realize a libelous or defamation lawsuit may be close behind if you don't back up all information. If you document anything within your work that could possibly slaughter a family member's character, be prepared for the backlash.
Interview family members, who can lend to the plot of your book.
Include family photos if possible. Readers like visuals and, if you can give them a clear picture of the family members you're talking about, this helps.
Fact check all information. Nothing will destroy the credibility of a book more than finding out that the information is wrong, especially in a nonfiction book.