Momo Sohma is Momiji's younger sister and notably the only member of the immediate family who isn't cursed by the Zodiac spirit. She was born after Momiji's mother had her memories of him completely erased—a cruel twist that allowed their father to start fresh and have a "normal" child. To protect that newfound happiness, he strictly forbids any contact between the siblings.
What's interesting is that Momo doesn't accept this boundary. She's drawn to Momiji, notices they look alike, and actively tries to find ways to learn about him. When she catches Tohru sneaking into the Souma compound, instead of ratting her out, Momo helps her get inside—and uses the opportunity to ask Tohru to pass along a message: does Momiji want to be her older brother?
The violin subplot reveals a lot about her. She started lessons, but when their father realized they might end up with the same teacher, he made Momiji quit to maintain the separation. Momo's real motivation only comes out later: she'd seen Momiji playing and wanted to learn so they could play together. When Momiji eventually learns that she's been asking about him and wants him in her life, he breaks down crying.
Momiji calls her cute, and she's typically glued to their mother's side—described as hovering "like a baby chick."
Content compiled by AnimeList.moe from publicly available sources.
