Complex sentence: Giffard's brother was Bishop Godfrey Giffard-the Bishop of Worcester and himself also Lord Chancellor of England; his sister Mabel was the Abbess of Shaftesbury.
Split into simpler sentences: Giffard's brother was Bishop Godfrey Giffard-the Bishop of Worcester and himself also Chancellor of England. His sister Mabel was the Abbess of Shaftesbury.