With his relative increase in strength, Seth hadn't needed to consume blood as much as he usually did.
As Seth saw it, Aimy was the closest opportunity he was going to get to feed on blood after his last meal from Minuha.
Deciding to use the opportunity to have a good drink, Seth innocently went about draining Aimy a little, but then just as he was starring, he felt Aimy's body tremble and go weak in his arms and then the unmistakable sound of tears reached his ears.
Seth was still confused about what was happening when he heard Irene's quaking voice.
"Seth, what are you doing?"
In Irene's voice, Seth could hear, fear, worry, and confusion, and it only took him a second to realize what was wrong.
Remembering the times when both sisters had seen him interact with someone's blood, Seth could only internally face-palm at his mistake.