Lost a brother in one sentence

Lin Tang lifted her little chin and left behind the words "Just you wait," as she entered the kitchen.

"...Heh, little thing's getting haughty now," Aiguo was amused.

He peeked into the kitchen again and saw his cousin handling things skillfully, which put his mind at ease.

Dusting off his clothes, he called his friend over to wash his face.

"Is that your sister? Your real sister?" Heizi glanced toward the kitchen and whispered.

The girl was really good-looking.

Nothing like Brother Aiguo's rugged face.

Aiguo immediately became alert.

Glaring at him like he was guarding against a thief.

He turned hostile on the spot, "If she's not my real sister, then what is she?"

"I'm warning you, don't even think about bad ideas with my sister, or we can't be brothers anymore."

Heizi, at the prospect of losing a brother over one sentence: "..."

"What idea did I have?" Heizi looked wronged.

All he did was ask a question; how did that become harboring ideas?