Complex sentence: While his paternal ancestors were landed gentry, whose holdings had once included Mitford Castle in Northumberland, his mother (Georgiana) Jemima was a daughter of the well-connected courtier the 3rd Earl of Ashburnham, with a very noble ancestry through the Earl of Beverley.
Split into simpler sentences: While his paternal ancestors were landed gentry, whose holdings had once included Mitford Castle in Northumberland. His mother (Georgiana) Jemima was a daughter of the courtier the 3rd Earl of Ashburnham, with a very noble ancestry through the Earl of Beverley.