1848-1868: Decades of Hell
The causes of the great upheavals, revolutions and wars of 1848 to 1868 were the pathetic attempts of the European Great Powers to stop the clock as it were, to stop the twin ideas of nationalism and liberty from growing powerful, this in itself caused by the nightmare that the French Revolution had caused. However, if the secret polices and armies they mustered were a great dam, then the revolutions would be the flood that swept all their defences away.
In 1820, the one-year old heir to the British throne, Princess Victoria, died of an illness, a tragedy that made Ernest Augustus of Hanover heir to the throne of Britain. This would be one of the key events that drastically expanded the scope of the 1848 revolutions.
However, many other events would lead to the utter disintegration of Europe's old monarchies in just one climactic year, and the great chaos that followed...
He is Heir AFTER his brother Frederick (Duke of York and childless) and his brother William (Duke of Clarence), and in 1820 after William's baby daughter who lived a few months
http://en.wikipedia.org/wiki/Princess_Elizabeth_of_Clarence
To simplify things, you could have Victoria die in mid 1821 when Ernest Augustus is thus Heir Presumptive after both of his brothers, noting of course that George III himself didn't die until 1820
IIRC historically Victoria was very ill aged 11, which would remove all of the other confusion since by that time Frederick was dead (1827) and George IV only had a year or so to live
Ernest Augustus would still be heir to his brother William in 1830, but by then it would be obvious and more easy to write a simple sentence about!
Best Regards
Grey Wolf