Xiao Yue did not consider herself to be a saint. She did not always do the right thing. She stole food, she picked on her brother and played pranks all the time. One time she even blamed a cousin for breaking Uncle Ma's favorite flower pot. Xiao Yue knew very well that she was not perfect.
But she could never, she would never, murder someone on her own accord. The shaman was a well-respected figure in her tribe. She must have her reasons. Uncle Ma trusted her. Uncle Ma was a reasonable, wise leader. He would not have agreed to a sinister plot without questioning it. All Xiao Yue had to do was... believe. In her people.
It's not like Long Zhao Feng demanded anything from her.