I think Su is a member of the Red Lotus
There’s a sibling betrayal thing going on lately. She at least knows more then she’s saying.
She is the right age to be involved with them.
We know she has criminal ties and background. And has no scruples of letting convicted criminals into her city.
In the Metal clan episode she mentions she was involved with a sand bender tribe and in the trailer where does Korra get ambushed at by the four red lotus members? The misty palms
Aiwei is her second in command, he could have lied when Suyin was interrogated by him. Her eyes shifted side to side when Korra first told her the news about Aiwei. How did Zaheer know Korra was there? Aiwei and Suyin told him. Suyin was quick to blame the young guard. Shared glances with Aiwei in the interagations.
Suyin sent Korra and her friends off alone to find Aiwei and willing lies to Lin about it.
How did she not know about that tunnel? Her husband is the architect and she is Toph’s daughter she should be able to use that awesome earth sense like Lin can.