1803: Jerome Bonaparte marries Elizabeth Patterson of Baltimore
1803-1804: Napoleon I is unhappy with the marriage, annulling it himself (since the Pope refuses). But Jerome is adamant that he will not put Elizabeth aside, and Napoleon eventually relents and admits them.
1805: Birth of Jerome Napoleon Bonaparte, son of Jerome and Elizabeth.
I think its unlikely that Napoleon would give Westphalia to Jerome given the tiff over Elizabeth; he'll likely hold a grudge similar to Lucien.
1812: Butterflies result in Napoleon deciding not to invade Russia, but choosing instead to consolidate his empire.
1812-20: Consolidation. The British make peace, with no prospect of overthrowing Napoleon. Napoleon has to make concessions in Iberia; withdraws from Portugal and Spain, except for the departments that were annexed to France, which instead become the new Kingdom of Catalonia for Joseph.
1821: Death of Napoleon I. His son, Napoleon II is crowned but is only 11.
1823: Jerome Napoleon Bonaparte, age 18, marries Crown Princess Zenaide of Catalonia, daughter of King Jerome (not impossible, she did marry Lucien's son in reality, albeit well after Waterloo; perhaps Jerome Napoleon is a favorite of Napoleon II, since Jerome Napoleon is likely to be raised in Paris, unlike his other cousins who are raised in foreign capitals).
1844: Death of Jerome, King of Catalonia. He is succeeded by King Jerome and Queen Zenaide.
Their descendants reign in Catalonia, which remains a part of the French sphere, initially as a vassal and later as an historic ally. Over time, in part due to government encouragement, there is a strong sense of Catalonian identity, so there is little impetus within the country to reunify with Spain.