I respectfully disagree. If, and it is a big 'If', her son murdered her daughter, it would not be totally out of the realm of human experience for her to lie to protect her son.
JonBenet was her rice-bowl. One does not break their own rice-bowl...
How can we judge her? The police, prosecutors and others poured over the evidence. Evidently it was not sufficient to implicate her. Consequently, I consider her not guilty.
Rice bowls are something to be treasured, not exploited in the way that poor little girl was. In JonBenet's brief life, she was never allowed to be a child. Her parents, especially her mother, personally saw to that.
I have no idea if any members of JonBenet's family were guilty in connection with her death, but there is an interesting Biblical parallel for the plight of a mother after one son has killed another one of her children in 2 Samuel 14.5-7. (That case is clearly signalled as a made-up story designed to elicit the desired reaction from King David.)