The Junius Brutus Alexander you refer to was born in Brooklyn, NY, 28 SEP 1867, to Junius Brutus Alexander, Sr., and Eliza H. Newcomb. He married Effie Shaw Emmons (21 AUG 1870-11 JAN 1899) on 25 JUN 1890.
Junius Brutus Alexander, Sr., was born 25 DEC 1814 in Prince William Co., VA, and died 9 JAN 1893 in NYC. He was the son of Gerard and Elizabeth Henry (Alexander) Alexander. He married twice; 22 DEC 1836 to Lucy Fitzhugh Dade (8 JUN 1816-15 JAN 1864), and 21 NOV 1866 to Eliza H. Newcomb (b. 13 SEP 1838).
Gerard Alexander was born 25 JUN 1784 at "Effingham" in Prince William Co., VA, and died 2 JUL 1834 in Meade Co., KY. His parents were Col. William and Sigismunda Mary (Massie or Massey) Alexander. His first wife was Eleanor Lee (13 AUG 1783-9 NOV 1807), dau. of Thomas Ludwell Lee. His second wife (m. 7 MAR 1810) was a cousin, Elizabeth Henry Alexander (28 JUN 1794-15 AUG 1860), dau. of Richard Barnes Alexander and Elizabeth Toy Whiting.
The lineage of the Alexander family of Virginia is well known and widely published and you should be able to pick it up using the information I gave you here. Feel free to email me if you have problems.