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.