He killed Tommy!

Prompt: Must include the line: “This isn’t over yet”
Entry for Flash fiction contest.

Sally bit his arm; he pulled her hair. She let go suddenly, twisting and putting a foot in his stomach pinching his arm.
“How dare you, you mean, ugly-”
“Let go of each other right now!” Mum had arrived to break up the fight.
Both children backed off, still bristling.
“He killed Tommy!” Sally wailed.
“It was just a stupid spider” Ben talked into the floor.
The two were ordered apart, to separate rooms and Mum went back to work.
Sally peered around the door and sent an aeroplane flying towards her brother’s head. She was gone before it hit.
Ben growled and unfolded the plane. “This isn’t over yet” said the writing. He knew she meant it; he was going to have to watch his back.
Ben was getting worried, his room was untouched, he checked all doors for traps before going through them, and his dinner had tasted fine, all through the meal Sally had smiled innocently at him making him nervous. He had no idea what she was planning. That night he jumped at noises for hours and dreamt of his sister smiling as sharks ate him alive.
Saturday morning the house was light and safe feeling and he could smell pancakes. The old kitchen table was already set out with plates, his parents and sister munching away. In his spot was a pile of small pancakes covered in cream and maple syrup, his stomach rumbled. As he sat down his sister gave him a radiant smile.
“I made them just for you” she said in a sweet voice.
Ben gulped.
“I’m not hungry at all.” He excused himself.
Sally cackled as she ate Ben’s pancakes. Her parents exchanged worried looks- they knew they’d missed something.
Ben looked up, Sally stood in his doorway.
“Revenge IS sweet.”

300 words.