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.
Did he have other siblings?
By Frances Pratt, Stewart's father had ten children who survived to adulthood.
How many passed as children?
CANNOTANSWER.
Did he marry?
In 1794, Stewart married Amelia (Emily) Hobart a daughter of John Hobart, 2nd Earl of Buckinghamshire, a former British Ambassador to Russia.
Did he father children with her?
By all accounts, the two remained devoted to each other to the end, but they had no children.
How long were they together?
the two remained devoted to each other to the end.

Question: Did he have other family?
Self-contained rewrite: Did Robert Stewart have other family, besides his father and mother?