I'm one of four siblings and I'm always fascinated by the varying ways in which we each remember some aspects of our family history. In my psychology / counselling / social work training this was an area that really interested me (often known as family scripts). Sometimes we'll be talking about an incident and discover that we have such different memories of who said what and in which order things happened, it's hard to be sure what actually took place. And yet each person is completely convinced that they're right.
We also have a divide in our upbringing, in that when my sister and I were young our parents were far less orthodox and observant, whereas my younger brothers were essentially brought up in a religious home. There are elements of my younger years that have been rewritten somewhat by older members of the family in a desire to establish a religious history. I find it a huge comfort that I have my sister to corroborate facts about what we did or didn't do before the light was seen.
When you marry or form a long-term relationship with another person, you're taking on board their history and their set of memories. You are two people who have your own trigger issues and comfort zones which are part of the scripts you've lived in your separate lives. Siblings-in-law then become the encyclopaedias and barometers of that history and you can see your spouse/partner as they are reflected in their own family back story.
In our case we also have a blended family on one side so there are people who kind of step into the script in act 2. They bring their own back stories and experiences which are then woven and intertwined with this family, forming something new. They see the dynamics as outsiders progressing to insiders over time. I suppose that's similar to marrying in. There is a value in all of the different perspectives, and trying to piece together the truth of a memory can be helped by hearing how a diverse group of connected people lived the same event.
I find families really intriguing and complex, and watching my children grow up and become their own distinct people is a privilege. I wonder how much is nature or nurture? I wonder what they'll argue about in 20 or 40 or 60 years time when they are telling their kids or partners about that time when x happened - how will their memories differ?