Complex sentence: Because she had become pregnant as a result of an affair following the disappearance of her husband, Read's mother began to disguise illegitimately born Mary as a boy after the death of Mary's older, legitimate brother Mark.
Split into simpler sentences: Because she had become pregnant as a result of an affair following the disappearance of her husband, Read's mother attempted to hide the birth of her daughter, Mary. She first began to disguise illegitimately born Mary as a boy after the death of Mary's older, legitimate brother Mark.