King Louis-Philipe's sons are a good example.
The elder, Ferdinand, married a mere duchess of Mecklenburg.
The second, Louis Dk of Nemours, married the daughter of the duke of Saxe-Coburg-Gotha.
Minor technical correction: Victoria was never the daughter of a reigning duke. Her father, Prince Ferdinand of Saxe-Coburg-Gotha, was the second son of Duke Franz of Saxe-Coburg-Saalfeld, who was a reigning monarch. He was succeeded by his eldest son, Duke Ernst I of Saxe-Coburg-Gotha, who in turn was succeeded by his own eldest and namesake, Duke Ernst II. The latter married but died childless, meaning that succession to the duchy eventually devolved upon the male issue of his younger brother, Prince Albert, who had married their mutual first cousin, the famous Queen Victoria of Great Britain.
But you're quite right that she (this namesake first cousin of the British queen) was of modest birth. In fact, her mother (the Hungarian Princess Maria Antonia Koháry) was not technically born to royalty but rather, nobility.
The third, François prince de Joinville, married the daughter of the Brazilian Emperor.
The fourth, Henri duke of Aumale, married the niece of the king of the Two Sicilies and
The fifth, Antoine duke of Montpensier, married the daughter of the King of Spain.
The 3 youngest married "better" than the two elder brothers.
Interestingly enough, the cousin marriages in the house of Orléans have merged the lines of the younger sons of King Louis-Philippe of the French, with those of the older sons. So marrying "better" turned out to be moot point, in the end.
What about Belgium ?
Wouldn't Albert, married to Paola, a Prince's daughter, have married better than Baudouin, Fabiola being just a marquess' daughter ?
Indeed, the younger son of King Leopold III married better than the older son; but once again, it didn't matter. It was no different from the situation in Liechtenstein, where the late Prince Franz Joseph II married married a mere noblewoman, kCountess Georgina von Wilczek, while two of his younger brothers married royals. One of them married an Habsburg archduchess (a descendant of Emperor Franz Joseph and Empress Elisabeth of Austria-Hungary) and another a duchess of Württemberg (sister of the present-day head of the house).
History repeated itself in the next generation, where the present-day sovereign (Prince Hans Adam II) is married to a woman of noble birth (Countess Kinsky of Wchinitz and Tettau), while one of his younger brothers (Prince Nikolaus) is married to Princess Margaretha of Luxembourg -- a daughter of Grand Duke Jean, who was the reigning monarch a;t the time.
None of this matters: if the marriage is dynastically approved, succession and rank in a reigning house are in accordance with the order of primogeniture.
But like I said, in non-reigning houses, one has witnessed issues arising from older sons marrying *unequally* (meaning women of common or noble birth), and hence morganatically, while younger sons married *equally* with women of royal birth. In Prussia, the royal succession eventually passed to Prince Louis Ferdinand, grandson of Kaiser Wilhelm II, who did not approve the marriage of his older brother to a commoner.
Duke Carl of Württemberg succeeded his father (Duke Philipp Albrecht) as head of the house, for reasons that his older brother (Duke Ludwig) married morganatically twice. And the present day head of the mediatized princely house of Leiningen, Andreas (married to Princess Alexandra of Hanover), succeeded because his parents disowned his older brother for marrying a commoner.