Believe your Mattie is the child of William L. Sibley and Margaret Graves. These Sibleys moved with the Dorsey family to Atkins, Pope Co., AR after 1880 but before 1892. they were living in Maysville, Issaquena Co., MS according to the census taken on 10 Jun 1880. William Sibley was the County Treasurer at that time. Census shows him to be 52 years old and Margaret was 40. Mattie was 13 and a sister whose name is unreadable was 11. I am still trying to find Margaret's ancestors. Her father was John William Graves.