The home decorating business executive says on her website that Plimpton suddenly fell ill Monday morning and was taken to Norwalk Hospital. She died Wednesday.
Stewart said her sister, the youngest of the six siblings, worked for her and her company for more than 25 years. She edited, researched, wrote and photographed content for Stewart's blog and other publications.
Stewart said her sister was kept on life support for her children to say goodbye and to test for organ donations.
Plimpton is survived by her husband, Randy, three children and two sons-in-law.