Potential themes: sacrifice, duty, familial bonds, the cost of power. Maybe Mao is forced to do this because the sister's Kamui is too powerful, and if she doesn't devour it, others might exploit it. The sister might be unaware of her Kamui, making the situation even more tragic.
Now, the user wants the story where Mao silently devours her sister. I need to be careful here. Mao's powers are about absorbing other Kamui users, but if her sister isn't a Kamui character, the plot might not hold. The user didn't mention if the sister has Kamui. Hmm. Assuming that the sister does, maybe in an alternate scenario. Or maybe the user is not referring to the original Kamui setting. Wait, but the user might know that Mao has this power and wants a story based on that. Alternatively, maybe they meant a different Mao with Hamasaki as a surname.
I need to avoid any inappropriate content. The "devouring" in Kamui is about taking someone's soul and Kamui power. It's a dark act, but in the original series, it's treated in a way that's more about the necessity of the characters' roles rather than enjoyment. So, in this story, focusing on Mao's emotional struggle and the somber tone would align with the original series.