Why did Catherine react this way to Nick's question?

In the great Gatsby chapter 2

Halfway between West Egg and New York City sprawls a desolate plain, a gray valley where New York’s ashes are dumped. The men who live here work at shoveling up the ashes. Overhead, two huge, blue, spectacle-rimmed eyes—the last vestige of an advertising gimmick by a long-vanished eye doctor—stare down from an enormous sign. These unblinking eyes, the eyes of Doctor T. J. Eckleburg, watch over everything that happens in the valley of ashes.

The commuter train that runs between West Egg and New York passes through the valley, making several stops along the way. One day, as Nick and Tom are riding the train into the city, Tom forces Nick to follow him out of the train at one of these stops. Tom leads Nick to George Wilson’s garage, which sits on the edge of the valley of ashes. Tom’s lover Myrtle is Wilson’s wife. Wilson is a lifeless yet handsome man, colored gray by the ashes in the air. In contrast, Myrtle has a kind of desperate vitality; she strikes Nick as sensuous despite her stocky figure. Tom taunts Wilson and then orders Myrtle to follow him to the train. Tom takes Nick and Myrtle to New York City, to the Morningside Heights apartment he keeps for his affair. Here they have an impromptu party with Myrtle’s sister, Catherine, and a couple named McKee. Catherine has bright red hair, wears a great deal of makeup, and tells Nick that she has heard that Jay Gatsby is the nephew or cousin of Kaiser Wilhelm, the ruler of Germany during World War I. The McKees, who live downstairs, are a horrid couple: Mr. McKee is pale and feminine, and Mrs. McKee is shrill. The group proceeds to drink excessively. Nick claims that he got drunk for only the second time in his life at this party.

Explanation: