Complex sentence: It was not until her early teens that she learned that the couple she thought were her parents were actually her grandparents, and that her mother, who as a young unmarried woman had almost given up her daughter to adoption, she believed was her sister.
Split into simpler sentences: It was not until her early teens that she learned that the couple she thought were her parents were actually her grandparents. Raitt believed that her mother, who as a young unmarried woman had almost given up for adoption, was her sister.