In-laws start married life at a disadvantage with their new family: they're lacking in all the family history that's gone before. I can remember one young man sitting with various members of our family and desperately trying to keep up with all that was going on, a great grin on his face to show he was involved, and the occasional comment to show he understood. But the reality was, for the most part he had no idea what it was all about, because after a while, families talk to each other in a kind of shorthand.
I've been in that boat myself, as no doubt all people have who've married into a family. And there's nothing you can do about it. You just have to pick it all up as you go along, and you may still be finding out things about your other family years after you've married into it. Some families are adept at hiding things almost forever, which doesn't help the 'out-laws,' as one of my fellow out-laws calls us. And then, when brothers and sisters get together, there's still a sense of exclusion for those who weren't born into the family.
Unfortunately it takes quite some effort on the part of a family to learn to include an in-law, and some families never make the effort. We've had to learn how to do it in our family and hopefully have made some progress.