The publishing world was shocked to learn that the gang-life memoir “Love and Consequences” was a fake. But even more startling was how that came to light.

The author, Margaret Seltzer, was exposed by her own sister.

While it isn’t clear why Ms. Seltzer’s older sister, Cyndi Hoffman, took on the role of whistleblower (neither sister returned phone calls), the incident throws a spotlight on society’s conflicted expectations of sisterhood. Even while criticizing Ms. Seltzer for her fabrication, some blog writers turned their ire on Ms. Hoffman, calling her a “tattletale” and speculating that she must have been jealous of her sister’s success.

“People were almost as fascinated by the fact that it was her sister as they were with the whole story,” said Marcia Millman, a sociology professor at the University of California, Santa Cruz, and the author of “The Perfect Sister: What Draws Us Together, What Drives Us Apart.”

“We have powerful expectations of loyalty from a sister,” she added. “But along with the idealized image of sisters, that they are always close, there is a stereotype that sisters are very competitive. It’s the two extremes.”