Because Yuan Shuyan was Yuan Yiyang's cousin and they lived under the same roof, Yuan Shuyan was surrounded by her classmates and asked about the shocking news.
Yuan Shuyan knew this cousin of hers wasn't a good guy, but she was also surprised by the crimes he had committed. However, the evidence was everywhere on the Internet now, and she couldn't avoid it.
Whether Yuan Yiyang really had done those terrible things, Yuan Shuyan refused to admit it was true. She kept on saying that it was just a set up, because it could seriously damage the Yuan family's reputation. However, all the evidence was public, so nobody believed Yuan Shuyan's words.
They chose to ask Yuan Shuyan about it after the evidence was public simply because they wanted to hear the answer from her mouth, but obviously they failed.