The 1815 Congress of Vienna had given the grand duchy to the house of Nassau, the rules of which were to govern succession in Luxembourg. As the law was semi-Salicism, Wilhelmina was not in a position to inherit the throne. Later on, however, the daughters of Grand Duke Guillaume IV were -- or not, acording to the Merenborgs, a morganatic branch of the Nassaus.
Queen Emma was a half-niece of Duke Adolf of Nassau-Weilburg, and wanted to see him acquire a throne after he lost his during the 1866 war between Prussia and Austria. Her action may have been a blessing in disguise, because his own descendants ended up in a minor dispute over the grand ducal succession, when Prince Georg (a half-cousin of Grand Duke Guillaume IV) tried to assert his and his son's rights to the throne of Luxembourg, despite the fact that his father (Prince Nikolaos) had predeceased his older half-brother in 1905.
They lost, of course, thanks to the 1907 Family Statute (which they disputed). But the point that needs to be stressed is that it took a special measure on the part of the grand duke to ensure the succession of his own daughters, as it was not clear that he was necessarily the last agnate of the house of Nassau.
He was the last DYNASTIC male, thanks to the morganatic marriage of Nikolaos; but as one can see in the Russian and Saxon succession disputes today, morganauts often have a hard time in accepting their non-dynastic status.