Marlon and Jayden, my younger two children, are both adopted, and our family has developed good friendships with the families who fostered them. We see Marlon's foster family about once a year on our way to or from camp, and we see Jayden's foster family every few months, even vacationing with them sometimes.
We have a similar situation with the birth family of the little girl we were fostering. Though she has gone home to them now, we have become friends with her parents and grandparents, and we go to visit her a few times a year, for Christmas and for her birthday.
Not all adoption and foster situations end up like this, of course, but the experience of coming into friendship with these other families has really challenged my definition of family. Jayden still refers to his foster parents as Mom and Dad, not with any sense of confusion about where he belongs, but because that is the role that they still play for him when we are together. All my children still refer to our foster child as their sister, and I will always feel like her father, not because I would want to replace her birth father, but because I was a father to her, even if only for a few months, and that is not a bond that can easily be broken.
What these experiences have shown me is that family is much larger than we often assume. Its definition is not with the nuclear family, not even with the biological family, but it spills out into all those who are willing to be family to each other. I think that this is something that all families could learn more deeply, whether they foster and adopt or not.