The date, of course, is questionable (see the note below). Even Edgar Poe made things more confusing when he once claimed Rosalie was actually his older sister.
Rumors persist that Rosalie Poe was not the daughter of David Poe. Evidence for when his estrangement begins is unclear but his wife having affair could certainly have inspired it, in theory. Even in Rosalie's lifetime accusations were implied. John Allan, for example, wrote to William Henry Leonard Poe (the oldest of the three children) in November 1824:
God may yet bless him [i.e. young Edgar] & you & that Success may crown all your endeavors & between you your poor Sister Rosalie may not suffer. At least She is half your Sister & God forbid my dear Henry that We should visit upon the living the Errors & frailties of the dead.The "Errors & frailties of the dead" would imply an affair that Eliza Poe had. How John Allan would have known about this is difficult to say though we might speculate that Eliza, a well-known and well-respected actress in Virginia, would easily fall prey to the gossip mill — whether rumors were true or not.
Allan also implies that Henry and Edgar need to care for Rosalie. Though she was (legally) adopted into the very loving Mackenzie family of Richmond, the story goes that she had some sort of mental deficiency. Some called her dull, others referred to her as "backward." These rumors contrast with stories that she made a modest living as a schoolteacher and enjoyed playing the piano and writing songs. Evidence for both versions of the story is unclear.
Rosalie likely was not too close with her brother Edgar, judging by their lack of substantial surviving correspondence. One letter from Edgar from sometime in early 1843 is addressed to one of the Mackenzies and notes, "Tell Rose I hope to see her before long, and that I will write to her soon." After Edgar's death, she continued to live with the Mackenzies, never marrying. The Civil War, however, "impoverished and broke up the family," writes Poe biographer Kenneth Silverman. Rosalie herself wrote she was forced "on the charities of this cold uncharitable world Homeless & Friendless."
She made her way to Baltimore, where she was rejected by Neilson Poe. Rumors suggest she walked the streets, hysterical and shrieking. She made money by selling postcards of her now famous brother and often appended signatures of her name with "Sister of Edgar A. Poe."
Rosalie eventually entered a shelter in Washington, D.C., where she died on July 21, 1874. Despite wanting to be buried near her brother (who would, one year later, be granted a substantial re-burying ceremony himself), she was instead buried in Washington in a plot belonging to the shelter. Her headstone oddly gives her year of birth as 1812 — a year after her mother's death.
* Though there is a claim that no evidence exists for this date, it actually comes from the Mackenzie family Bible. However, even the Mackenzie's weren't entirely sure. They wrote: "Rosalie Mackenzie Poe... (is said) was born 20 Decr. 1810." The date has been supported, even with a question mark, by Poe scholars including Arthur Hobson Quinn and the two authors of The Poe Log.