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

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