Sam patiently waited her out, hands curled around his half-forgotten coffee. The word vessel brought up about a million complicated emotions, connected to half-formed thoughts, but it had nothing on the way he felt about the rest of it. The idea of a sister driven mad by possession of the Mantle hit far too close to home, and the idea of her taking action against Sarissa like that was heartwrenching.
Because that was what he was afraid of, wasn't it? That the Mark, the demonic blackness of Dean's soul would eventually twist him out of control, maybe even to the point where he would try to take Sam down with him. The thought of that was unbearable, and it took Sam a moment to put it aside in order to concentrate on what Sarissa was saying. Even still, he felt it keenly enough that her pain was reflected in his expression.
"That's awful," he said. And then, shaking his head, "Awful doesn't even begin to cover it. I'm sorry."