The fic opens when Katiya, mermaid princess of the Bering Sea, approaches Lucia and her friends for aid in rescuing her little sister Kyla, who was captured by fishermen while they were out playing and has been put on display in an aquarium. Together with Katiya, Lucia, Hanon, and Rina mount a rescue mission.
The Warmth of a Sister's Love contains examples of the following tropes:
- Affectionate Nickname: Kyla often calls Katiya "Kat" for short.
- Big Sister Instinct: Katiya goes to quite a few great lengths to save Kyla, beginning with swimming all the way to Japan to get help from Lucia and her friends.
- Flashback Nightmare: In the first few chapters, Katiya ends up having nightmares about Kyla's capture by the fishermen.
- Humans Are Bastards: Katiya is a firm believer in this to the extent that she actually hates it when she has to assume her human form. However, Lucia and the others, having lived amongst humans for far longer than Katiya has, are quick to point out that not all humans are bad, and by the end, Katiya has started to accept this.
- Token Good Teammate: Johnny, a member of the crew of fishermen who caught Kyla in her net. He actually tries to persuade his crewmates to let her go, and later helps Katiya and the other mermaids break her out of the aquarium.