I have a very intriguing question about The Song Of The Sea film. Why isn't the brother a selkie? I have already looked up on Google. I even researched the folklore story of the selkie. I just don't understand how his sister was a selkie but they have the same mother, who was also a selkie.
The answer comes from the status of selkies in folklore. Selkies are just one of significant class of shapeshifting, sexual nature spirits (such as merrows, swan maidens, and, more distantly related, dryads who ensnare men that venture too deep into the forest primeval) that may mate with or marry ordinary men.
These marriages are typically conducted via trickery—although depending on tale, the trickery may be on the part of either the male or the female partner. A selkie may sneak on land, disguised as a human woman, and marry a fisherman or other land dweller, bearing him children, until he discovers her true nature, and she is obliged to return to the sea. Or the man may capture a selkie (or other shape-changing water spirit) by stealing the magical animal skin or garment that she uses to change form; in that version of the story, she flees back to the water when she recovers the lost garment, whose location is often revealed by her children.
Male analogues of these sexual wildings typically appear in a different kind of story. While a dryad captivating a man in the deep woods represents the danger of getting lost in the wilds, a male nature spirit, like Heer Halewijn or a neck, represented the danger of death, often due to drowning, for a woman out on her own. A woman captured by a male elf-knight might be raped before being murdered, but unless she (like lady Isobel in Child ballad 4) managed to trick and kill her captor, she would not live long enough to bear his offspring.
So, since the selkie-type is an intrinsically female role, the offspring of a human father and a selkie (or other such spirit) mother cannot include a male selkie. The pair's sons are either obvious half-human, half-animal monstrosities, or if they appear normal, they are simply ordinary human males. Or, in some versions, their children may be exclusively female. The daughters, on the other hand, are typically the same kinds of creatures as their mothers. Depending on the circumstances of the mother's departure, the daughters may accompany their mothers, or they may remain with their fathers, only departing when they reach maturity. This represents the situation in the film.