I see this a lot in writing workshops. Someone comes in and sets out the long-term goal of writing their memoirs or some autobiographical work. Personally, I always support this, because it gives the writer a chance to examine their life and explore the long and winding path they've traveled. The importance of their journey relative to others isn't even a concern. What matters is that they are setting out to write about the world they have seen. That is also when the tough part begins.
On paper, I had some pretty standard-issue parents. Despite different educations and backgrounds, their paths crossed in 1960 working at the same place in Chicago. They got married, had kids, changed jobs, bought a house in the suburbs, and divorced in the 1970s - pretty much the American Story. Their families were scattered about, they had parents who were my grandparents, and all these other relatives who were my aunts, uncles, and cousins. This is a lot of information relevant to my life, but very few details I have offered really participate in the story that is my life. Describing these parts is like describing the colors in a painting - it is part of the whole work, but we need to see how they are used to understand what the artist wants to present.
Instead of dumping all the information on you about the shape and size of my family, let's see how I perceived them and how their presence (or absence) affected me. After all, an autobiography should be a very personal experience, and what is more personal than viewing all those family details from the eyes of the person who lived it.
Example: I did have the standard family set-up on paper. However, three of my grandparents had passed away by the time I was four, and my remaining grandmother was in her 70s and not very active in my life. If the reader wants to learn about me, they need to learn about how this influenced my vision of the world, so I need to offer that. I saw the world as a place where grandparents were old, rare, and distant. Because of this, it was a treat to see a grandparent, but also kind of sad because those visits only lasted maybe for the weekend, a few times a year. It was also difficult for me to understand how I somehow had friends with living great-grandparents, and three or four living grandparents, some much younger than seemed possible. At this point, what I am telling you is my story, not the story of my existence.
Documenting the details of one's existence is not very difficult, and is definitely a way to pass information on to future generations. However, this is different than telling your story - what you saw, what you experienced, and what you felt. Your story can be amazing, adventurous, and even scary at times, simply because it is how you saw the world at a particular time.
More importantly, it is the story that nobody else can write other than you. The world from your eyes and your mind is something nobody else has experienced, and it's the best story you can write.