"Sister Princess" centers around Wataru Minakami, a young boy who, after failing his high school entrance exam, is sent to a mysterious island by his father. Upon arrival, Wataru discovers that he has twelve sisters, each with distinct personalities and traits, who all adore him and refer to him as their "onii-chan" (big brother). The sisters—Aria, Karen, Mamoru, Marie, Haruka, Kaho, Sakuya, Rinrin, Yotsuba, Shirayuki, Chikage, and Hinako—live together in a large mansion, and Wataru is expected to take on the role of their older brother.

The narrative unfolds in a slice-of-life format, focusing on Wataru's daily interactions with his sisters as he adjusts to his new life. Each sister has her own quirks and interests, ranging from Aria's love of music to Karen's passion for cooking and Mamoru's fascination with science. The series explores their relationships with Wataru, highlighting moments of warmth, humor, and occasional misunderstandings. While the premise initially appears lighthearted, subtle hints and mysteries surrounding the island and the sisters' origins add an undercurrent of intrigue.

Wataru's character is portrayed as kind-hearted and protective, though he often finds himself overwhelmed by the sisters' affection and the responsibilities of being their older brother. The sisters, while deeply devoted to Wataru, also exhibit individual growth as they navigate their own dreams and challenges. The series balances episodic storytelling with overarching themes of family, belonging, and the bonds that unite siblings.

The island itself serves as a serene and idyllic backdrop, with its picturesque landscapes and tranquil atmosphere contrasting with the occasional emotional or dramatic moments. The narrative avoids heavy conflict, instead focusing on the gentle, heartwarming dynamics between Wataru and his sisters. While the series does not delve deeply into the origins of the sisters or the island's mysteries, it leaves room for interpretation, allowing viewers to engage with the story on a personal level.

Overall, "Sister Princess" is a character-driven series that emphasizes familial love and the joys of sibling relationships, set against a backdrop of mystery and charm. Its episodic structure and focus on character interactions make it a comforting and engaging watch for fans of slice-of-life anime.