The answer to this question can be found in Chapter 16, after the party that is held for Angela's forthcoming wedding where she unwraps a bomb or a firework that explodes in her face leaving her with a wound on her cheek. What Turtle whispers to Angela after their parents have left is that she knows it was her sister who is the bomber and who planted the fireworks in that package so that when she opened the present they would explode in her face. Turtle therefore thanks her sister that she pulled the package towards her face rather than allowing it to explode in Turtle's face. Note what Turtle says to Angela:
"I brought your tapestry bag; I didn't look at your notes or clues, honest." But she had removed the incriminating evidence.
Turtle obviously finds evidence that shows it was Angela who purchased the fireworks and then wrapped them up before opening them, showing without a doubt that she is the bomber everybody is looking for.