The 13th Marquess was the eldest of five sons of David Hay, 12th Marquess of Tweeddale (1921-1979), and his first son (and elder twin son) by his first wife Hon.
Since none of the three brothers (sons of the 12th Marquess's first marriage) are married, the next in succession are their two half-brothers, sons of the 12th Marquess's second marriage.