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Male Sylvester Rockwell Click to view Sylvester Rockwell in the family tree

Sylvester was born after 1753.1   Sylvester's father was Samuel Rockwell and his mother was Mabel Mehitable Loomis.  His paternal grandparents were Thomas Grant and Elizabeth Rockwell; his maternal grandparents were Moses Loomis and Elizabeth Bidwell. He had two brothers and two sisters, and also a sibling of unknown gender, named Gustavus, Thomas, Tryphena, Elizabeth and Wereham.  He was the third oldest of the six children.1 

1 phelps.FTW @ Tree #0105
Further sources/citations:
World Family Tree Vol. 3, Ed. 1 @ Tree #3669, 3669.ftw