How does Catherine in The Great Gatsby show loyalty to her sister and save her good name?

Myrtle's sister Catherine knows all about Myrtle's affair with Tom Buchanan. It is always Catherine whom Myrtle uses as an excuse for going into New York, and the "love nest" shared by Myrtle and Tom is in the same building as Catherine's apartment. At the coroner's inquest, however, Catherine shows loyalty to her sister and saves her good name by giving completely false testimony. Nick Carraway obviously attended the inquest, or inquests, into the causes...

Myrtle's sister Catherine knows all about Myrtle's affair with Tom Buchanan. It is always Catherine whom Myrtle uses as an excuse for going into New York, and the "love nest" shared by Myrtle and Tom is in the same building as Catherine's apartment. At the coroner's inquest, however, Catherine shows loyalty to her sister and saves her good name by giving completely false testimony. Nick Carraway obviously attended the inquest, or inquests, into the causes of the three deaths, but he may not have been called upon to testify. In Chapter IX, Nick speaks approvingly of Catherine for forestalling a big media scandal.

[She] swore that her sister had never seen Gatsby, that her sister was completely happy with her husband, that her sister had been into no mischief whatever.

Since the three principals in the tragedy, Gatsby, Myrtle, and her husband George, were all dead, this left Catherine as the most knowledgeable witness at the inquest. According to Nick, the coroner's jury decided that Myrtle's husband was a man "deranged by grief."