I find the edge of shadows is a great place for available light portraits. Archways, the edge of buidings and large trees all work great. Also as Suzanne said windows work great. Subjects are everywhere. You missed on these sisters but there is someone waiting around the next bend. People for the most part really enjoy being part of the experiance. just make sure you share your thoughts through or during the process of creating their portrait and share images with them afterwards. Most of all be flexibly in charge. Have a plan, even if it changes the confidence of some control remains and gives both you and the sitter a goal. Lastly really learn to look at light. Very important!. Either let the light dictate the pose or create light to match the pose but either way make an effort to understand how it is describing your portrait.