Complex sentence: But, his paternal grandmother, Sally Dunning, was at least the third generation of free people of color; in the 1860 census, she is listed as a free mulatto, along with her mother, grandmother, and siblings.
Split into simpler sentences: But, his paternal grandmother, Sally Dunning, was at least the third generation of free people of color. In the 1860 census, she is listed as a free mulatto, as were her mother, grandmother, and siblings.