What is the moral lesson of "Roman Fever"?

The moral lesson of "Roman Fever" is that often one misjudges a person who is close.  In the exposition of Wharton's story, Mrs. Slade, with dramatic irony, remarks, "Grace Ansley was always old-fashioned."  And, while Mrs. slade notices that Mrs. Ansley says, "[Rome] it's still the most beautiful view in the world" with an emphasis on me, she does not understand why.

As the two friends talk, they wonder about their girls.This time with knowing iirony, Mrs. Ansley remarks to her companion,

...I don't in the least know what they are...And perhaps we didn't know much more about each other.

Mrs. Ansley does not know as Mrs. Slade thinks, "Would she never cure herself of envying her?"  Mrs. Slade has always been jealous that her husband was once in love with Mrs. Ansley.  In a vengeful moment when Mrs. Slade reveals that it was she who wrote the letter inviting Mrs. Ansley to the Coliseum, Mrs. Slade believes there is a "slow struggle  behind the voluntarily controlled mask of her small quiet face."  With dramatic irony, she calls Mrs. Ansley "prudent."

Of course, Mrs. Ansley was anything but prudent as she met Delphin Slade and became pregnant from their personal Roman fever.  But, having been told that Mrs. Slade had written the letter that she has so long cherished, Mrs. Ansley realizes that Mrs. Slade has told her about the letter because she has continued to hate her.  But, it is the final truth that sets Mrs. Slade back to her bitter envy:  Barbara, whom she has always wished were her daughter, is actually the daughter of her husband and her old friend.  Mrs. Slade has known so little of her "intimate friend" because she has always remained her rival.