Disney did this on purpose

And they're both gingers!

Surely Hercules is her uncle?

Sammy!

Hold the fuck up, I need to put my head in my hands an think about this for a second

Ariel is the daughter of Triton.
Triton is the son of Poseidon.
So Ariel is the grand-daughter of Poseidon.
Poseidon is the brother of Zeus.
So Ariel is the niece of Zeus.
Zeus is the father of Hercules.
So in correction, Ariel isn't the cousin of Hercules, she is his great-niece. O.O

Hercules is Triton's cousin, not Ariel's

They're second cousins