Conversation so far:
What were Robert Stewart, Viscount Castlereagh's parents like?
Robert Stewart, Viscount Castlereagh's father, the Robert Stewart senior, was an Irish politician and prominent Ulster landowner.
Who was Robert Stewart, Viscount Castlereagh's mother?
Robert Stewart, Viscount Castlereagh's mother, who died in childbirth when he was a year old, was Lady Sarah Frances Seymour-Conway, daughter of Francis Seymour-Conway, 1st Marquess of Hertford
Did Robert Stewart, Viscount Castlereagh have other siblings?
Charles Vane, 3rd Marquess of Londonderry's half-brother from his father's first marriage was Robert Stewart, Viscount Castlereagh.
How many of Robert Stewart senior's chidlren passed as children to adulthood?
By Frances Pratt, Robert Stewart, Viscount Castlereagh's father had ten children who survived to adulthood.
Did Robert Stewart, Viscount Castlereagh marry?
In 1794, Robert Stewart, Viscount Castlereagh married Amelia Hobart a daughter of John Hobart, 2nd Earl of Buckinghamshire.
Did Robert Stewart, Viscount Castlereagh father children with Amelia Hobart?
By all accounts, Robert Stewart, Viscount Castlereagh and Amelia Hobart remained devoted to each other to the end, but they had no children.
Did Robert Stewart, Viscount Castlereagh have other family?
The elder Robert Stewart, Viscount Castlereagh's marriages linked his family with the upper ranks of English nobility and political elites.

Question: Did he have cousins?
Self-contained rewrite: Did Robert Stewart, Viscount Castlereagh have cousins?