First, I need to confirm if the sister is part of the original Kamui universe or another setting. If the sister is another Kamui holder, then Mao consuming her could be a natural progression. However, in the original story, Mao does absorb her own mother and some others. She might absorb her sister if the sister is a Kamui user.

Mao’s nights were plagued by visions of Suki’s future: her sister trapped, her spirit writhing under the Red Arms’ control. The Kamui code was clear—devour a Kamui before it falls. Mao’s hand trembled when she touched Suki’s shoulders at their farewell dinner, the girl’s warmth a balm against her resolve. "Stay away from Tokyo," she murmured. "Go to Odaiba." A lie. She needed Suki close, but vulnerable, so their moment would be private.

Suki’s Kamui now coursed through Mao, blooming like chrysanthemums in winter. The Red Arms retreated, unaware of their missed target. Yet, Mao carried a hollow ache, the memory of Suki’s laughter a thorn in every breath. She wore the new power like armor, a reminder of the cost of survival.