Mary F. Rose, born about 1832 in Lunenburg, Virginia, the daughter of Archibald Rose and Elizabeth Mason Holloway. Mary was the sixth of Archibald and Elizabeth's seven children. Her elder sister, Louisa, was married at 18 to Henry Gary, a widower twelve years her senior. Henry was already the father of five; he and Louisa they had two children together - Mary, born 1845, and Thomas, born 1850, before her death about 1851. Her sister Mary then married Henry before 1853. They had two sons, Hartwell (Henry) in 1853, and Charles in 1856. Mary was 21 at the time of her marriage; Henry was 45. While she did inherit her sister's children with her marriage, Henry's older children were either grown or living with their mother's parents.
I've come across several instances of women marrying their sister's widower. It seems to have been a common practice at the time, I suppose in the interest of making sure children were cared for, and keeping property and money in the family. It also seems to have been a family tradition; Louisa and Mary's mother, Elizabeth Holloway, had been married to a James Rose before her marriage to Archibald. It doesn't appear that Archibald and James were brothers, but I'm certain there is a family connection. Since marriage was traditionally not necessarily a romantic relationship, and women had few rights to property or options for living outside of marriage (or under the roof of a male relative), it makes sense that they would choose to make a marriage that would allow them to keep whatever property rights they may have had, as well as making sure their sister's children were well taken care of.
Mary outlived Henry, who died between 1860 - 1870. In 1870, she is living in one stepson's household, and in 1880, in another. She doesn't appear on any census records after 1880, so she presumably died some time after that date.