Jenny is quiet and refined. She is the daughter of dynamic and powerful Mrs. Slade. Mrs. Slade had always wanted a daughter who was "brilliant" but got one who was "an angel."
Barbara is bright and vivacious and full of spark and energy; she fits the description "brilliant." She is the daughter of quiet, unobtrusive, mild Mrs. Ainsley.
It turns out that Mrs. Ainsley once had a tryst with Delphin Slade, husband of Jenny's mother, Mrs. Slade. At the moment of truth in the story, Mrs. Ainsley confesses that she "had Barbara" from her tryst with Delphin. This confession identifies Delphin Slade as Barbara's father.
Mrs. Ansley was again silent. ...
"I had Barbara," she said, and began to move ahead of Mrs. Slade toward the stairway.
In language of ambiguity, Wharton's description of Jenny Slade suggests that Mrs. Delphin Slade had a rendezvous of her own with Mr. Ainsley. If so, parallel circumstances exist and Jenny Slade is actually the daughter of Mrs. Ainsley's husband, Mr. Ainsley.
If this idea of the crisscross of daughters is part of the revelation, then each woman has raised the daughter of her rival's husband--Mrs. Ainsley having done so happily and Mrs. Slade having done so with some regret.
[Mrs. Slade]: "There must be times... but there! I always wanted a brilliant daughter... and never quite understood why I got an angel instead."
If this is correct, then it is Jenny--with her quiet alluring ways--who would pose a threat to Barbara. Wharton emphasizes that Mr. Ainsley and Mrs. Ainsley are almost identical matches in the temperament and personality. Theoretically, even if Mrs. Ainsley weren't her mother, Jenny would still have the needed traits to charm a man away from a "brilliant" Barbara. An indirect third generation of man-snatcher may arise from a new family connection.
Should Jenny be wary of Barbara? I should think not. The ability to overthrow a previous attachment is all in the power of the quiet, modest ones: the great-aunt and the present Mrs. Ainsley. Therefore, one would expect Barbara would need to wary of the silently charming Jenny, not the other way round.
I agree, and that is the answer I was looking for. Barbara has more to fear from Jenny than vice-versa, but it is intriguing that Roman Fever as a weapon has a stance in both families.