What was she laughing at?
At them, presumably.

Katharine had risen, and was glancing hither and thither, at the presses and the cupboards, and all the machinery of the office, as if she included them all in her rather malicious amusement, which caused Mary to keep her eyes on her straightly and rather fiercely, as if she were a gay-plumed, mischievous bird, who might light on the topmost bough and pick off the ruddiest cherry, without any warning.

Two women less like each other could scarcely be imagined, Ralph thought, looking from one to the other.

Next moment, he too, rose, and nodding to Mary, as Katharine said good-bye, opened the door for her, and followed her out.
Mary sat still and made no attempt to prevent them from going.

For a second or two after the door had shut on them her eyes rested on the door with a straightforward fierceness in which, for a moment, a certain degree of bewilderment seemed to enter; but, after a brief hesitation, she put down her cup and proceeded to clear away the tea-things.
The impulse which had driven Ralph to take this action was the result of a very swift little piece of reasoning, and thus, perhaps, was not quite so much of an impulse as it seemed.