Final answer:
The situation King Triton (presumably a mistaken reference to King Cepheus) comes to understand likely pertains to the wickedness of his family, specifically Queen Cassiopeia, and the need to sacrifice his daughter as a direct consequence of that vanity and pride.
Step-by-step explanation:
The full gravity of the situation that dawns on King Triton, as per the given context, is B) The wickedness of his sister. The emphasis on Poseidon's creation of a sea-monster, Cetus, to wreak havoc on the kingdom due to Queen Cassiopeia's vanity, indicates the narrative explores themes of pride and divine retribution. This context does not refer directly to 'King Triton', which seems to be an incorrect character reference from 'The Little Mermaid' and might be a typo, where the correct character implicated seems to be King Cepheus. However, as King Cepheus realizes the necessity of sacrificing his daughter to appease the angered deity, it underlines the direness inherent to his situation and the severe consequences of his family's actions.