Complex sentence: Her brother was Robert Beaton, 3rd Laird of Creich, and her sister, Elizabeth Beaton was a mistress of King James V of Scotland, by whom she had an illegitimate daughter, Jean.
Split into simpler sentences: Her brother was Robert Beaton, 3rd Laird of Creich. Her sister, Elizabeth Beaton was a mistress of King James V of Scotland, by whom she had an illegitimate daughter, Jean.