Poe's younger sister was christened "Rosalie Mackenzie Poe" in Richmond on September 3, 1812. I've discussed Poe's older brother here before, but this is the first (and, likely, the only) post on his sister.
Rosalie is a bit of an enigma; even her birth is unknown (believed to be about December 20, 1810). Rumors persist that she was mentally deficient in some way, though it's unclear if these reports are just another attempt at slandering Poe.* Rumors also suggest that she was not the daughter of David Poe, Jr., father of Edgar and William Henry Leonard Poe, but that she was a bastard child, born after mother Eliza Poe had an affair with another man (inspiring David to abandon her).** While Eliza was on her deathbed, her children, including Rosalie, were tended to by fellow actors with the last name of Usher. After Eliza's death, the Mackenzie family of Richmond took in her youngest daughter and legally adopted her (something that the Allan family never did for Poe).
When William and Jane (Scott) Mackenzie took little Rosalie in, they already had two children (a third, the first-born, died within his first year). They eventually would have ten children in addition to the little one they adopted. Only a couple days after her christening, a woman reported to Frances Allan (Poe's foster-mother) that "Rosalie is not expected to live." Despite the pessimistic outlook, Rosalie survived that illness and, in fact, went on to outlive her brother by 25 years.
Rosalie attended a girls' school in Richmond run by Miss Jane Mackenzie, sister of adoptive father William Mackenzie. Although they grew up in the same town, it is unclear if Poe saw his sister often in his early years (though the Allan family and the Mackenzie family did correspond). In 1842, she traveled to Philadelphia to spend a week with her blood brother (he called her "Rose")e. They corresponded from then on, Poe giving her updates on his wife Virginia's health and she requesting autographs for friends. In 1843, the Mackenzies built a home they named Duncan Lodge, where Poe visited a few times in 1848 and 1849. Rosalie became either a teacher or a teacher's assistant at Miss Jane's school for about nine years. We know she played the piano; the Richmond Poe Museum still has the piano on which she played.
Rosalie was raised in relative wealth. However, after the Civil War, the family fortune was squandered by one of her brothers-by-adoption. Rosalie should have been the legal heir to Poe's published works after her brother's death. Instead, somehow his works were given to Rufus Griswold, either by mistake, by trickery, or by Maria Clemm (Poe's mother-in-law). Rosalie made some attempts, under the advice of a lawyer named J. R. Thompson, to claim those rights. The result was a bit of a family quarrel between Rosalie and Maria Poe (who was also her father's sister). Griswold seems to have also stepped in to push Rosalie aside.
Instead, Rosalie became a desperate beggar, occasionally supported by fans of her brother's work. She survived as best she could by selling portraits of her brother the famous author. In her 60s, she was admitted to a shelter in Washington, D.C. There, she died on July 21, 1874. She had expressed a hope to be buried by her brother. Instead, she was buried at Rock Creek Cemetery in a pauper's grave owned by the home.
* Some state these rumors as fact. Jeffrey Meyers reports she was "a dull, tedious, pathetic figure, [who] failed to develop mentally after the age of twelve" and that Poe was "repelled" by her. Earlier, biographer Joseph Wood Krutch called her a "harmless imbecile."
** One such accusation came from John Allan, Poe's less-than-ideal foster-father (perhaps ironic, considering Allan had his own bastard children). It doesn't seem impossible. Either way, Poe biographer William Bitter blames Allan's rumor-mongering for Rosalie never getting married.