THank you, Johan. Very interesting and helpful commentary. I agree that Nicholas II may have regretted forbidding Grand Duke Michael's marriage to Princess Beatrice of Edinburgh. I find it interesting that there were so many Orthodox-Protestant marriages among royal families in the nineteenth century, given the great differences between say, the Russian Orthodox faith and the Lutheran faith. There would probably be more common ground between High Church Anglicans and the Orthodox, but Queen Victoria was more Low Church, so I imagine she was not a big fan of Orthodox ritual, just as she was not a fan of Catholicism. It seems that the children were generally raised in the faith of the father, or at least that is the pattern I see in the various Orthodox-Protestant royal marriages with which I am familiar. Catholic-Protestant and Catholic-Orthodox marriages were both fairly rare, though I can think of a few examples (e.g., Prince Waldemar of Denmark and Princess Marie of Orleans, with the boys being raised Protestant and the girl Catholic; William IV of Luxembourg and Infanta Maria Anna of Portugal, with all the girls being raised Catholic).
The Russian orthodox church may have forbidden first cousins marrying but Willem III, both Sophie's (named for their grandmother Sophia Dorothea) and Karl-Alexander all were protestants and not members of the Russian Orthodox faith.
Anna Pavlovna did object to the marriage of her son with his first cousin but that had probably more to do with the fact that Sophie of Wurttemberg was the daughter of Catherine Pavlovna who had been the favourite daughter of her mother and the favourite sibling of the worshipped older brother Alexander I.
She did not object to her daughter marrying a first cousin and never commented on the first cousin marriages of her husband's two siblings.
Her sister-in-law princess Marianne of the Netherlands was first engaged to prince Gustav the son and heir of king Gustav IV of Sweden who had been replaced by his uncle Carl XIII who had been succeeded by Carl XIV Johan (formerly known as Jean Baptiste Bernadotte). Bernadotte had the protection of the Russian Tsar and could continue his reign. Anna was very dismissive of Marrianne's fiancee and was happy when the engagement was broken off. In spite of the fact that Gustav was the son of Alexander I's sister-in-law Frederica of Baden. Very wealthy and had a good reputation.
Shortly afterwards Marianne was married to her maternal first cousin Albert of Prussia. A marriage that did not end well. She did not accept her husband's infidelity and was forced to leave the realm. When she got pregnant by her lover a quick divorce was arranged.
The Russian orthodox rules were not that strict. Since Peter the Great the Tsar was also the Head of the Russian Orthodox church. That gave the ruler the freedom to accept things normally not allowed.
The marriage of Grand Duke Cyrill to Victoria Melita of Saxe-Coburg-Gotha/Edinburgh was accepted (the issue for him getting angry about it at first had everything to do that Victoria had divorced Ernst of Hesse-Darmstadt the brother of Nicholas II's wife Alexandra Feodorovna).
At the same time he did not consent to a match between his brother Grand Duke Michael and another of the Edinburgh-girls (Beatrice). He probably lived to regret that decision. Had Michael and Beatrice been allowed to marry and produced healthy sons he would have had a brother to support him in his uneasy role and heirs other than the despised offspring of his uncle Wladimir.
What is interesting in these three examples is that the Russian Orthodox Church (I believe) forbids first-cousin marriages, but that the children of these three Russian Grand Duchesses did in fact make first cousin marriages. I wonder what their mothers/maternal relatives thought about this? Did the Greek Orthodox Church also ban first-cousin marriages, or was this purely a feature of the Russian Church?
The oldest and youngest from Anna Pavlovna's and Willem II's children married a maternal first cousin.
The oldest (Willem III) married Sophie of Wurttemberg daughter of his mother's deceased sister Catharina Pavlovna, queen of Wurttemberg.
The youngest (Sophie of the Netherlands) married Karl Alexander of Saxe-Weimar-Eisenach son of Maria Pavlovna an other of the 6 daughters from Paul Petrovich marriage to Maria Feodorovna (born Sophia Dorothea of Wurttemberg).
I know that Infanta Isabel of Portugal (granddaughter of Ferdinand and Isabel of Spain) married her first cousin, Holy Roman Emperor Carlos V. A more recent example of maternal parallel cousins marrying would be the Grand Duchess Charlotte of Luxembourg and Prince Felix of Bourbon-Parma. Can anybody name other examples in history of couples whose mothers were sisters?
As for agnatic cousins, I'm sure there are plenty of more examples. Such unions have occurred everywhere, from the UK (e.g. grandchildren of Frederick, Prince of Wales), Wuerttemberg, and Bavaria, to name just a few examples. Can anybody provide a list of these as well?