Several reasons come to mind that would have drawn Jennifer and Amanda together and kept them together over the years. Being neighbors put them in a position to interact with each other frequently. They were both strong, intelligent women in professions of service to others. The older, childless Amanda had a nurturing tendency to Jennifer's children, especially Fiona, and somewhat to Jennifer and James. Amanda and Jennifer could disagree, argue, even fight, but they could also forgave each other. They respected each others strengths.
What kept them together in the long run? They really did know too much about each other to let the relationship dissolve, didn't they?