Complex sentence: They had a son, Thomas, and a daughter Genet, who married Sir Thomas Fitzwilliam and had four children: they were the grandparents of Thomas FitzWilliam, 1st Viscount FitzWilliam.
Split into simpler sentences: They had a son, Thomas, and a daughter Genet. Genet married Sir Thomas Fitzwilliam and had four children: they were the grandparents of Thomas FitzWilliam, 1st Viscount FitzWilliam.