King Gustaf IV Adolf of Sweden reigned 1792 to 1809.
Suppose he is King until 1837. Suppose he and Queen Frederica have only their daughter, Princess Cecilia (born 1807). Cecilia is recognized as the heir to the throne.
In 1831 she still marries Augustus, Grand Duke of Oldenburg as his third wife.
Their son, Gunther Friedrich Elimar Gustaf is born in 1835. His elder half-brother Peter is the heir to the Grand Duchy of Oldenburg.
In 1837 Cecilia becomes Queen Regnant of Sweden. What happens then?