Who defines a family as "relationships by blood, marriage, or affection, in which members may cooperate economically, may care for any children, and may consider their identity to be intimately connected to the larger group"?

Karen Seccombe is the correct answer.

Karen Seccombe was the author of the book "Revel for Families and Their Social Worlds". In the book she talks about how her perspective and definition of families has broaden up. While growing up, her definition of families matched with the one provided by the U. S Census Bureau and now her definition is way more accurate on the dynamic of today's families (the definition you have just mentioned).