Jack - I believe that Ann's bio page mentions the book "Survey of Ishams in England and America", so evidently they're a family that's been written about.
I would assume her siblings had children but could not say. Her family maintained a home in southern Vermont, where her father's business partner, Robert Todd Lincoln also had one (the Hildene estate, which was owned by the Lincolns into the 1970's and is now a reception hall and museum), and where my family has long had one. Ann's father was first president of one of the country clubs there and some local history books mention the family. Never seen anything specifically mentioning nieces and nephews.
This will probably be of interest to no one but me, but I see from her death notice in the NY Times that her memorial service was held in the Congregational Church in Manchester, Vermont; the beautiful church where my sister was married. It's right across the street from the famous Equinox Hotel, where Mary Todd Lincoln frequently stayed. It's always seemed to me that the Ishams and Lincolns were close friends in addition to business associates, if they had their second homes in the same place. Or perhaps Edward Isham and Robert Todd Lincoln were simply workaholics.
When I visited Grace Church in New York to see the Edith Evans memorial, I noticed the name Isham on a plate on one of the pews, so Ann's brother may have been a member of the same church as Edith Evans. Interesting!