Still it's not all that new. At the end of the 18th century the Stadholder prince Willem V of Orange had three children. A daughter Louise and two sons his heir Willem Frederik, hereditary prince of Orange and prince Frederik.
After the heir married his Prussian first cousin by birth an HRH the stadholder suggested it would be better if his younger son would not marry a Royal princess but an ebenburtig lady from a lower rank than a Royal house. So the two sisters-in-law would not get in to a potential conflict about status and style.
The younger prince did fall in love with a Royal princess (princess Mary daughter of George III) but as he was exiled at that time he was in no position to propose to her. He went into service of the Austrian army and died shortly afterwards in Italy. But it already shows that even in the 18th century rulers were thinking about the status of younger lines and how rivalry with the senior line might not be desirable.
Interesting ... I've had discussions with a friend on the conflicts that can potentially arise, whenever a younger brother marries better than an older brother. In fact, it is believed that the principle of Ebenbürtigkeit (which is neither ancient nor universal) evolved in part because of this trend, in royal history. Eventually, it became the practice in some houses whereby the marriage of the said older brother was morganatic, if the wife clearly was of unequal rank at birth.
Otherwise, one simply had tensions arising -- as you outline in the Dutch case -- from the fine points of distinction between the royal and non-royal rank, even among *equally* born wives within a given court. I know, for a fact, that the Duchess of Edinburgh (wife of the second son of Britain's Queen Victoria) resented the low rank in the royal court of her adopted land. In Russia, she was the only daughter of Czar Alexander II, and a born Imperial Highness, the Grand Duchess Maria Alexandrovna.
Not only was she outranked by a *mere* Royal Highness in the UK (the Princess of Wales was a daughter of Christian IX of Denmark, who was *only* a king, not an emperor: he didn't even come to the Danish throne until 1863, eight years after Alexander II came to the Russian throne) but also, she was outranked by her sisters-in-law (the younger daughters of QV). She outranked them only when in the presence of her husband, the Duke. And to add insult to injury, she was obliged to style herself as a Royal and Imperial Highness -- not Imperial and Royal Highness.
Two generations later, one witnessed similar tensions in the British royal court, where the Duchess of York (born the Honourable Elizabeth Bowes-Lyon) outranked the Duchess of Gloucester (born Lady Alice Montague-Douglas-Scott, daughter of the 7th Duke of Buccleuch and Queensberry), who in turn outranked the Duchess of Kent (born HRH Princess Marina of Greece and Denmark). The latter was known to make snideful remarks about "those common little Scots girls my husband's brothers married."
I'm not sure about the situation in Bavaria, where the eldest son (the future King Ludwig III) of Prince Luitpold (who would eventually be named regent of the kingdom) married an Habsburg archduchess from a cadet branch of the imperial dynasty (the deposed house of Austria-Este, where her uncle Francesco V was the last reigning duke of Modena), while his second son (Prince Leopold) married an Habsburg archduchess from the reigning imperial line (a daughter of Emperor Franz Joseph). Maria Theresia, I believe, was actually born to *only* the royal (not imperial) rank, while Gisela was certainly a born Imperial Highness. I'm not sure about relations between the two women, and the question of rank in the Bavarian royal court.
Certainly matters were far less complicated with the marriage of Luitpold's youngest son, since his wife was born a mere Serene Highness, Princess Theresa of Liechtenstein (albeit a daughter of one sovereign prince and sister of two others). If it came to that, Leopold originally fell for Princess Amalie of Saxe-Coburg and Gotha (Kohary). It's just that Empress Elisabeth had intended for her as the bride of her youngest and favorite brother, Duke Max Emanuel in Bavaria, who had fallen in love with her himself and confided his feelings to his older sister.
Sissi adored her baby brother and desired his happiness. So to distract Leopold and divert his attention from the beauteous Amalie, she arranged a meeting between him and her own daughter Gisela. Evidently the prospect of being a son-in-law of the Emperor Franz Joseph was enticing enough for him to accept the invitation and propose -- once he received the cue that he would be welcomed as a prospective husband for the girl.
The handsome dowry he received from his father-in-law did much to help him overcome his earlier infatuation with his first love, who evidently returned his affections and was crushed by the *betrayal* (unaware of the machinations of the Empress). But the meeting between Amalie and Max Emanuel passed off successfully, and the two wed happily. So all's well that ends well ...
That being said, had she and Leopold wed, one would have had a neat situation in the Bavarian royal court, where the eldest son of the prince regent was married to a born Royal Highness, the second son was married a born Serene Highness, and the youngest son was also married a born Serene Highness. Certainly Princess Amalie of Saxe-Coburg and Gotha (a granddaughter of King Louis-Philippe of the French) would have been just as acceptable as a dynastic bride (deemed *equally born*) for a prince of Bavaria, as a reigning emperor's daughter -- even according to the Wittelsbach house laws at the time. As it was, things had to be interesting with the marriage of Leopold and Gisela (who by all accounts had a happy union, just like Max Emanuel and Amalie).
But in an enthroned house, as long as the marriage is dynastic, the wife of an older son always outranks the wife of a younger son. After all, it's easy to lower marital standards if the monarchy is still reigning. Matters are trickier in non-reigning dynasties, whose house laws are not easily amended. We have seen cases of morganatic marriages involving elder sons in royal houses like Prussia and Württemberg, among other cases.