Robert Stewart was born in Henry Street, Dublin, Ireland, in 1769 the son of Robert Stewart (1739-1821) of Newtownards and Comber in County Down, with properties in Counties Donegal and Londonderry. The family seat was Mount Stewart, County Down.  The elder Stewart was an Irish politician and prominent Ulster landowner He was created Baron Londonderry in 1789, Viscount Castlereagh in 1795, and Earl of Londonderry in 1796 by King George III. In 1771 he was elected in the Whig interest to the Irish House of Commons, where he was a supporter of Lord Charlemont and his allies who called for greater independence from Britain. From the Act of Union of 1800, however, he sat in the British House of Lords as an Irish representative peer. In 1816 he was created Marquess of Londonderry by the Prince Regent.  Stewart'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 (a former British Ambassador to France (1764-65) and Lord Lieutenant of Ireland (1765-66)) and Isabella Fitzroy, daughter of Charles FitzRoy, 2nd Duke of Grafton. His father remarried five years later to Lady Frances Pratt, daughter of Charles Pratt, 1st Earl Camden (1714-94), a leading English jurist and prominent political supporter of both the 1st Earl of Chatham and his son, William Pitt the Younger. Through the elder Stewart's marriages, he linked his family with the upper ranks of English nobility and political elites. The Camden connection was to be especially important for the political careers of both him and his elder son. By Frances Pratt, Stewart's father had ten children who survived to adulthood, including Stewart's half-brother, Charles William Stewart (later Vane), Baron Stewart of Stewart's Court and Ballylawn in County Donegal (1814) and 3rd Marquess of Londonderry (1822).  In 1794, Stewart married Amelia (Emily) Hobart a daughter of John Hobart, 2nd Earl of Buckinghamshire, a former British Ambassador to Russia (1762-65) and Lord Lieutenant of Ireland (1776-80). Her mother, Caroline Conolly, was the granddaughter of William Conolly, Speaker of the Irish House of Commons in the early 18th century and one of the wealthiest landowners in Ireland. Caroline's brother, Thomas Conolly, was married to Louisa Lennox, sister of Emily FitzGerald, Duchess of Leinster, whose son and Emily's cousin-by-marriage, the aristocratic rebel Lord Edward FitzGerald, was a leader of the United Irishmen and one of their martyrs in the early stages of the Irish Rebellion of 1798.  Emily Stewart was well known as a hostess for her husband in both Ireland and London and during some of his most important diplomatic missions. In later years she was a leader of Regency London high society as one of the Lady Patronesses of Almack's. She is noted in contemporary accounts for her attractiveness, volubility and eccentricities. By all accounts, the two remained devoted to each other to the end, but they had no children. The couple did, however, care for the young Frederick Stewart, while his father, Stewart's half-brother, Charles, was serving in the army.

Answer this question "Did he have other siblings?" by extracting the answer from the text above.
By Frances Pratt, Stewart's father had ten children who survived to adulthood,