Complex sentence: Ellen was so light-skinned that she was often mistaken for a member of her master's family, which irritated her mistress so much that she gave the eleven-year old Ellen as a wedding gift to one of her daughters, Eliza.
Split into simpler sentences: Ellen was so light-skinned that she was often mistaken for a member of her master's family. This irritated her mistress so much that she gave the eleven-year old Ellen as a wedding gift to one of her daughters, Eliza.