Determine the story you wish to tell. Unlike a family history, a family memoir is written more as a narrative, rather than a catalog of pertinent information. Pare down all the details of your family history to focus on one significant event. This will prevent you from airing your family secrets unnecessarily.
Decide who is going to be your audience. Perhaps you are doing this privately, for yourself, to heal yourself from past pain. Perhaps you are doing it for your children, so they can know the rich family history they share. This will set your tone for your memoir and will give you a framework as to which details you will need to include, and how explicit you need to be.
Negotiate with your family, when possible and appropriate. Talk to your family about your intention to write a memoir for posterity, and feel them out for what information they would like to include. Perhaps, they will instruct you to leave out specific information or encourage you to explore more.
Change names, settings or situations, if you plan to make the memoir public. This will reveal the darkest secrets only to those who would have been originally privy to them. Truth is relative when you begin writing a memoir, as your perception of events will drive the narrative, more than facts and statistics.
Don't be afraid to expose flaws and weaknesses. The story of a perfect family that always loved each other and never quarreled would never sell to any reader, including yourself. Worse, it would wreak of insincerity. Be brave enough to be honest.
Use your own life experience to consider the perspective of your family members, especially those for whom you may carry a grudge. By empathizing, you will be able to understand them better, which will allow you to more effectively tell their story. Most of all, this will build a bridge between you, rather than burn one by using the memoir as written revenge.