As a general rule, if there is a king and no queen consort, the queen dowager is the first lady of the kingdom. This was true of Queen Mary in the reign of Edward VIII
While Emma was regent Wilhelmina was a minor so there was no point of outranking the other. The moment Wilhelmina turned 18 on august 31st 1898 she was HM the Queen and her mother was no longer the regent but HM the dowager queen Emma, she was however most often referred to as the HM the Queen-mother.
Is it a given that in a kingdom, the highest-ranking lady in the land is always the enthroned queen (meaning either a queen regnant or the consort of an enthroned king) -- assuming there is one? The exception, I believe, would have been Russia, where according to protocol (as promulgated by Czar Paul) a dowager empress was to always outrank the empress consort.
This, of course, was a sore point for Czarina Alexandra Feodorovna, consort of Czar Nicholas II -- notwithstanding the fact that she was by nature a shy and reserved person who should have been happy to yield the spotlight to a polished and experienced person such as her mother-in-law.
It was an equally sore point for Queen Alexandra of Great Britain, whose sister was that same dowager empress Maria Feodorovna, who (influenced by the example of her own Romanov imperial court) encouraged her to try to retain her premier rank at the British royal court, even after the death of her husband (King Edward VII) and accession of her son (King George V). But "when in Rome, do as Romans do": was Russia the exception, where an enthroned queen could be outranked by a dowager queen?
Of course, a newly ascended king might not, for the time being, have a queen. Did the newly dowager Queen Mary (widow of King George V) retain her premier rank at the British royal court, during the brief (11-month) reign of her eldest son (King Edward VIII)? If so, I would assume that she was obliged to yield that rank to her daughter-in-law Elizabeth, consort of the newly ascended King George VI, later on in the year 1936 ...
Similarly, I was wondering if dowager Queen Elisabeth of the Belgians regained her rank as the premier lady of the land, after the untimely death in 1935 of her daughter-in-law, Queen Astrid ... As it was, her son (King Leopold III) remarried only morganatically, in 1941. So his second wife, born Lilian Baels, not only did not receive the title of Queen but also, would most certainly not have outranked her mother-in-law ... But certainly nobody questioned the premier rank of Queen Fabiola, who became the consort of King Baudouin in 1960. I believe that Elisabeth (by virtue of being a queen, if only a dowager) would nonetheless have outranked the then-Princess Paola (who had married Baudouin's younger brother the preceding year).
In today's world, nobody questions that Queen Mathilde outranks her mother-in-law, Queen Paola, wife of an abdicated king. Ditto for Queen Letizia of Spain. Earlier in Spanish royal history, Queen Maria Cristina outranked her mother-in-law, notwithstanding the fact that she had been a queen regnant. But the deposed Isabel II had been forced to abdicate in favor of her son, King Alfonso XII, and been obliged to accept that her daughter-in-law was named regent of the kingdom after his death. Dowager Queen Maria Cristina, however, was obliged to yield to her own daughter-in-law (Queen Ena) later on. I believe she largely retired from public life in 1906, when she lost her rank as the premier lady in the land.
As for Queen Emma of the Netherlands: did she actually outrank her daughter, Queen Wilhelmina, during the remaining years of her minority, during which time she served as regent of the kingdom? I understand that a regent exercises all the constitutional powers and functions of a sovereign. Although nominally a queen regnant, Wilhelmina was only a child of 10 at the time of her father's death in 1890: she did not get officially enthroned until obtaining her legal majority, in 1898. The position of Queen Maxima today would be comparable to that of Queen Maria Cristina of Spain, in the 19th century: the difference is that Queen Beatrix voluntarily stepped down (just like her Benelux counterparts), as opposed to being forced to abdicate the throne.
Of course, a kingdom could theoretically have no queen whatsoever -- regnant, consort or dowager: that was the situation in Norway, between the death in 1938 of Maud and the accession of King Harald V in 1991. It has been said that Princess Astrid (younger daughter of King Olaf V) became the "unofficial first lady" of the land, after the death in 1954 or her mother, Crown Princess Martha. I assume that she lost that premier position in the Norwegian royal court after the arrival of Crown Princess Sonja in 1968 -- correct?
Bavaria also underwent a long period without a queen -- between the death in 1889 of Marie (consort of King Maximilian II and mothers of Kings Ludwig II and Otto, neither of whom having ever married) and the accession in 1913 of King Ludwig III. Marie had, in fact, been the only queen of Bavaria for the last 35 years of her life, as her mother-in-law died in 1854. Since Queen Therese was the wife of an abdicated king (Ludwig I) from 1848, she would have been outranked by her daughter-in-law for the last six years of her life -- correct?
Does this mean that Marie was the premier lady of the land for those 41 years during which time she was the queen consort, and later dowager queen? How did the other Bavarian royal women rank at court? Three of her sisters-in-law (Mathilde, Adelgunde, and Hildegarde) had left Bavaria to marry foreign royals; only Princess Alexandra stayed in the land and never married. The deposed and exiled Queen Amalia of Greece set up, with her husband (King Otto) a *rival* Greek royal court in Munich, after being sent back to Bavaria in disgrace (1862). Archduchess Augusta Ferdinanda of Austria-Tuscany died in 1864 (only a few months after the death of her brother-in-law, King Maximilian II), and Infanta Amelia Filippina of Spain arrived in the Bavarian royal court only in 1856.
As for the next generation (meaning the children of Prince Luitpold) and members of the ducal branch of the house of Wittelsbach ... it's too complicated a question to raise here, but perhaps somebody could explain ...