Sparr is Galen and Urik's half-brother
While it is made clear that Urik, Galen, and Sparr all share a mother, Queen Zara, their paternity is not noted in the books. Seeing their physical features and the age gap between Sparr and his older brothers, it may be that Zara married one man, had Urik and Galen, her husband died, then she remarried to a merman-like person "hence Sparr's fins." Sparr having a different father than Galen and Urik is clear from his physical features. Urik, Galen, and Zara are all round-faced, with straight hairlines and brown or blonde hair. Sparr has a narrow face, a hooked nose, and slicked-back black hair with a widow's peak. Black hair and widow's peaks are very dominant traits, meaning Sparr either had a grandparent with those features, or, more likely, seeing his fins and him being much younger than his brothers, came from a second marriage.
Relna and Galen are related
Relna and Keeah have a strong connection to Eric and his house, as Eric discovers in one of his dreams. In female form, Galen's name is Gelna, a name very close to Keeah's mother's. Seeing Galen and Urik's features, one can deduce that their father had wavy blonde hair and round features rather similar to Relna's. If Relna and Galen share paternal ancestry, this would make Eric and Keeah distant cousins, making their bond have a familial source.
Even before they fully manifested, Neal's genie powers were responsible for his frequent shapeshifting
A three-eyed bug monster, a goblin, a turtle, etc.
Dark Magic has mutagenic effects
The more of it you use, the less human you become. Hence Sparr's fins and Demither, who presumably started out as a human based off the humanity of her sister Queen Relna and niece Keeah, turning into a thorny-haired mermaid.
