And even if your Mom didn't work outside the home, she certainly had enough to do to take care of the family and the house. Step-grandparents can play the exact same role in a child's life as grandparents do. It does not matter how you are related. It does not matter where love and patience and nuturing come from. Children need stability.
Grandparents probably live in the same house for a long time.
They may even have the same car, furniture, and pets for years. Even if grandchildren don't visit very often, familiarity is comforting. Older people tend to be more routine-oriented, and children like agendas. They feel safe knowing what will happen at Grandma and Grandpa's house if it's basically the same every time they are there. Each of us is a product of our experience and the combined influences of people we knew when we were growing up.