Conversation so far:
What were Robert Stewart, Viscount Castlereagh's parents like?
The elder Stewart was an Irish politician and prominent Ulster landowner He was created Baron Londonderry in 1789.
And his mother?
Stewart's mother, who died in childbirth when he was a year old, was Lady Sarah Frances Seymour-Conway, daughter of Francis Seymour-Conway.

Question: Did he have other siblings?
Self-contained rewrite: Did Robert Stewart have other siblings, besides himself?