So when doing some digging around this morning, I was surprised to learn that Sanger's father Michael was ardently in favor of George's ideas. My surprise was only in the individual,(my reaction was more like "Oh wow - now, that figures. It makes perfect sense.") for in the aggregate it is impossible to have progressive ideology without Henry George.
The Higgins family was so impacted by the work of George that one of Margaret's brothers was named Henry George McGlynn Higgins. This is just as significant for what you do see - as what you don't see. Henry George McGlynn Higgins, is named both after Henry George himself, and also a well known(I would say notorious) at the time Georgist priest in New York, Edward McGlynn.
In Margaret Sanger: A Life of Passion, the following is written: (page 18)
Similarly intolerable to most of Corning was Michael Higgins's support of Henry George's radical solution to the inequitable distribution of wealth in America. George proposed a single tax for landowners on the unimproved value of land. In a home with few books, George's exposition of this idea in Progress and Poverty - Published in 1879, the year of Maggie Higgins's birth - held an important place in a small family library that included the Bible, Aesop's Fables, Gulliver's Travels, Thomas Moore's Lalla Rookh, and Michael's medical books on physiology.
Indeed, here is what Margaret Sanger herself had to say about her brother, who passed away at the young age of four: (Autobiography, page 29)
Henry George McGlynn Higgins had been named for two of the rebel figures father most admired.
Henry George strikes again. Henry George is to progressive ideology what Karl Marx is to communist ideology.