I don't think its Seina going rage mode that killed their mother since we do see Fumina witnessing it first hand back when she was a literal child, so its seperate character and since she's the older sister, Seina might've been a baby at that point. (This is from Chapter 9.5 - The Volume 1 extras)
I don't think its Seina going rage mode that killed their mother since we do see Fumina witnessing it first hand back when she was a literal child, so its seperate character and since she's the older sister, Seina might've been a baby at that point. (This is from Chapter 9.5 - The Volume 1 extras)