This episode was during Angela's wedding shower, when she was receiving unwanted gifts from a lot of her mother's friends:
"Get away!" Angela cried, jerking the gift up and away from her sister as the lid blasted off with a shattering bang. Bang! Bang! A rapid ra-a-tat-tat. Rockets shooting, fireballs bursting, comets shrieking, sparks sizzling. Two dozen framed flower prints falling off the wall. (96-7)
Angela pushes her sister away from the gift just in time, because Angela knows that the gold-wrapped gift is the bomb. The only way she could have known that was if she was the bomber herself. Turtle figures this out, and lets her sister know that she'll keep her secret. She talks to Angela, who is getting her bombing injuries tended to in the hospital:
"Just thanks. The fireworks would have gone off in my face if you hadn't pulled the box toward you. ....But, wow, that sure was a dumb thing to do." (100)
Angela was so desperate to escape marrying Denton Deere that she set the bombs herself. She couldn't just talk to her mother and tell her that she wanted to go back to college rather than become a young bride. This was her dysfunctional way of derailing her mother's plans.
Source: Raskin, Ellen. The Westing Game. New York: Avon Books, 1978.