Well, MrGreene, it's about time somene acknowledged the important role of the women. Great!
Mama Corleone was the heart of the family. She represented the tie with Sicilian ideals of family and parenthood. As long as she was alive, she stood for, "you can never lose your family," no matter what. That's one of the reasons Michael didn't whack Fredo until after she died. When she died, the only person in the world who could have kept that ideal alive in Michael died.
But Kay was influential in her own way. She was, to use a hackneyed phrase, Michael's "better half." She was the American ideal of "legitimacy." Although Michael's nature pulled him toward gangsterism, Kay constantly set within him a kind of dynamic tension between the dark side and what he wanted to be, even if he could't be.