John Stewart, 1st Earl of Lennox
John Stewart, 1st Earl of Lennox( before 14308 Jul/ 11 Sep 1495) was known as Lord Darnley and later as the Earl of Lennox.
Henry Stuart, Lord Darnley
Henry Stuart (or Stewart), Duke of Albany (7 December 1545 – 10 February 1567), styled as Lord Darnley until 1565, was king consort of Scotland from 1565 until his murder at Kirk o' Field in 1567.
Many contemporary narratives describing his life and death refer to him as Lord Darnley, his title as heir apparent to the Earldom of Lennox, and it is by this appellation that he is now generally known.
He was the second but eldest surviving son of Matthew Stewart, 4th Earl of Lennox, and his wife Lady Margaret Douglas.
Darnley's maternal grandparents were Archibald Douglas, 6th Earl of Angus, and Margaret, daughter of Henry VII of England and widow of James IV of Scotland.
He was a first cousin and the second husband of Mary, Queen of Scots, and was the father of her son James VI of Scotland, who succeeded Elizabeth I of England as James I.
Margaret, Countess of Lennox
Margaret, Countess of Lennox was the daughter of Domhnall, Earl of Lennox, who died in about 1364.
She married Walter de Fasselane, a cousin, who also descended from previous Earls of Lennox.
Margaret and Walter's children were:
Esmé Stewart, 1st Duke of Lennox
Esmé Stewart, 1st Duke of Lennox, 1st Earl of Lennox( 154226 May 1583) was the son of John Stewart, 5th Lord of Aubigny, who was the younger brother of Matthew Stewart, 4th Earl of Lennox, and his wife Anne de la Queuille.
Sir James Melville described him as" of nature, upright, just, and gentle".
James Stuart, 2nd Earl of Bute
James Stuart, 2nd Earl of Bute( before 1696 – 28 January 1723) was the son of James Stuart, 1st Earl of Bute and Agnes Mackenzie.
Elizabeth Stuart, Countess of Lennox
Elizabeth Stuart, Countess of Lennox née Cavendish (31 March 1555 – 16 January 1582) was an English noblewoman and the wife of Charles Stuart, 1st Earl of Lennox.
She was the mother of Arbella Stuart, a close claimant to the English and Scottish thrones.
Matthew Stewart, 4th Earl of Lennox
Matthew Stewart, 4th Earl of Lennox( 21 September 15164 September 1571), was the fourth Earl of Lennox, and a leader of the Catholic nobility in Scotland.
He was the son of John Stewart, 3rd Earl of Lennox and Lady Elizabeth Stewart, daughter of John Stewart, 1st Earl of Atholl.
His grandson was King James VI of Scotland and I of England.
Charles Stuart, 1st Earl of Lennox
Charles Stuart, 1st (or 5th) Earl of Lennox (April or May 1557 – April 1576), was the fourth son of Matthew Stewart, 4th Earl of Lennox and Margaret Douglas, daughter of Margaret Tudor and granddaughter of King Henry VII of England.
His brother was Henry Stuart, Lord Darnley.
Robert Stewart, 1st Earl of March
Robert Stewart, 1st Earl of Lennox then 1st Earl of March( c. 152229 March 1586) was a Scottish nobleman of the family of Stewart of Darnley.
Charles Stuart, 1st Baron Stuart de Rothesay
Charles Stuart, 1st Baron Stuart de Rothesay (2 January 1779 – 6 November 1845), known as Sir Charles Stuart between 1812 and 1828, was a British diplomat.
He was twice Ambassador to France and also served as Ambassador to Russia between 1841 and 1844.

Question: Who is the sibling-in-law of Charles Stuart, 1St Earl Of Lennox?
Answer: Mary, Queen of Scots