Nitpick: The present Earl Nelson is not a descendant of the famous Admiral Horatio Nelson (who had no legitimate children), nor even of the 1st Earl Nelson (Horatio’s brother William, whose legitimate issue also went extinct). Instead he is a descendant of the 2nd Earl, Thomas Bolton (nephew of Horatio and William), who changed his surname to ‘Nelson’ when he was earled.
Allowing an Earldom to be inherited by a brother, let alone via a sister, was a special favor, but no posthumous honour was too great for the man who won the Battle of Trafalgar. The same special remainder was specified when the Admiral was given his 2nd Barony in 1801, after winning the Battle of Copenhagen. Thus the 6th Earl Nelson of Trafalgar and of Merton was also the 7th Baron Nelson of the Nile and of Hilborough.