OP here, I wanted to provide additional context for the specific aspect I'm asking about. The following comes from WorldHistory.com's article on Mélusine.

Link with Noble Houses

In all these tales [of the supernatural wife], the man is at fault for breaking a taboo or a trust, and the same is true in the Mélusine legend. The difference in this tale is that the men seem to understand the futility of trying to win their wives back, Mélusine though they clearly loved them, adding a tragic dimension to the story and elevating Melusine as a heroine undeserving of her fate.

The legend was purposefully revised by Jean d’Arras between c. 1387-1393 for political purposes to make Mélusine more sympathetic as the foundress of the Lusignan Dynasty. The basic elements of the story itself are not that different from many others of its type, however, as noted by scholars Maria Leach and Jerome Fried:

"The breaking of the taboo, reduplicated in this story by the incident of Pressine, [Mélusine's mother], is itself simply another version of the offended supernatural wife motif told by others and worked into this tale so that it would not be lost. Parallels of the Mélusine story are found in several parts of the world." (705)

D’Arras altered the offended supernatural wife motif to make sure an audience would understand Mélusine as a good Christian beloved by her subjects who was unjustly cursed by a hard-hearted mother and betrayed by a faithless husband. The work was commissioned by Jean, duc de Berry of the House of Lusignan best known as a patron of the arts and collector of illuminated manuscripts. An image of Mélusine as a dragon, flying over the fortress of Lusignan, appears in the famous illuminated manuscript Les Très Riches Heures du Duc de Berry (begun c. 1412-1416) commissioned by the duke.

The duke requested the Mélusine [descent] tale from d’Arras as part of a propaganda campaign. The story ends in d’Arras’ version with Mélusine returning to the castle of Lusignan in 1374, just before the duke’s forces liberated it from the English. Her cries signaled the duke’s victory and the castle changing hands, establishing the duke as the rightful heir of Mélusine, who was thought to have magically built the fortress when she was first with Raymondin. In d’Arras’ version of the story, then, the duke is linked directly to the founder of the Lusignan house, establishing his legitimacy.

The Duke of Berry is only one noble to claim descent from Mélusine, however, as she was also claimed as an ancestor by the nobility of Britain, Germany, and Luxembourg. Richard I of England (r. 1189-1199) claimed Mélusine as a direct ancestor, supposedly boasting of his relationship with a supernatural entity he felt brought him luck. Other noble houses similarly linked themselves with the legend at least 200 years before Jean d’Arras wrote his version of Mélusine, and she still appears on the family heraldry of a number of European noble families.

Why the nobility of medieval, Catholic Europe chose to link themselves to a supernatural entity who, in all the versions of the story, does not attend Mass, is a question scholars still debate and discuss. The 14th-century French writer Couldrette links Mélusine to the Arthurian legends in his Roman de Mélusine through the figure of the Good Knight of England, a member of King Arthur’s court, who goes in search of the treasure of Mélusine's father, [the English/Scottish nobleman or King Elinas; in some versions, the King of Albania]. He fails in his quest, however, as it has been decreed that Mélusine's son, Geoffrey of the Great-Tooth (also "Geoffrey Big-Tooth"), will defeat the treasure’s guardian, and no other.

As the Arthurian Legend was embraced by a number of Europe’s nobility, Mélusine came to be regarded as belonging to each by their respective writers. Although a supernatural entity, she is given Christian values and virtues by most if not all writers of medieval literature, and her children were regarded as heroic foundational figures by the various houses of Europe accordingly.

Also another question from this: Who were Mélusine's ten children with her human husband, Raymondin of Poitiers (France, House of Lusignan), and how did nobles and royals trace their ancestry to her directly? [Other versions also put Mélusine's husband as Count Siegfried of Luxembourg, for the claim by the House of Luxembourg, but the story is otherwise identical.]