Yours, Mine, and Ours

My niece slept over last night. I loved having her here. I love her curly hair and her high pitched giggle. I love how she joins in on the laughing, teasing, and pillow fights that go on here. But most of all, I love how comfortable my kids are at my sisters' houses, and how comfortable their kids are at my house. Yes, my kids are mine, and their kids are theirs, but in a way, all of them are ours. We help each other a lot, and as a result, our children spend lots of time together and are all growing up as best friends, from the oldest down to the smallest. It's good to know that as they grow up, they each have a whole posse of brothers and sisters and cousins that are there to help them. Is it just my family that's lucky this way, or is this true in most families?