His mother was Louise Boyce who grew up on Ulster Plantation, now Boyce Louisiana. His father, Powhatan, was a professor at LSU, moving to Baltimore after the Civil War. Pohwatan had a sister, Elizabeth.
After graduating West Point, he joned the U.S. Cavalry in the West as a part of the Indian Campaigns.He headed a troop that came to be known as the Buffalo soldiers.His bravery during one of the campaigns led to his receipt of a Congressional Medal of Honor.
Don't know about the Samuel Clemens relationship. I know there is no relation on his mother's side.
Powhatan's grandafther was Judge Henry Boyce, a successful lawyer, judge, planter in central Louisiana, near Alexandria.