Hanging in her parlor was the "lost" Thomas Sully painting of Maria Cecil Gist Gratz, beloved wife of Benjamin Gratz, Rebecca's younger brother.
And why shouldn't it be there? Roberts is Maria and Benjamin's great-great-great-granddaughter. She also owned a pastel copy of the Sully painting.
Roberts immediately e-mailed Judith Guston, Rosenbach curator, who had planted the blog post in the long-shot hope that the portraits would be found - some day.
It took a couple of weeks.
"I told her I might have what they were looking for," Roberts recalled.
They spoke on the phone.
"How amazing is that?" Guston said Wednesday. "When I got the call, I almost fell out of my chair."
The Rosenbach has been accumulating Gratz family materials for decades, following in the collecting footsteps of the museum's founders, book dealers A.S.W. and Philip Rosenbach, who were Gratz family relations.
Two years ago, the museum acquired an 1831 Thomas Sully portrait of Rebecca Gratz, and a portrait of her brother Joseph, by G.P.A. Healy.
The paintings joined an ensemble of Gratz images at the Rosenbach, including another Sully painting of Rebecca and one Sully did of her brother Benjamin, Maria's husband.
The Sully portraits were all painted in Philadelphia in 1831 at virtually the same time.
But no one knew what had happened to a fourth painting Sully executed in 1831 - of Maria.
Guston combed through auction and estate records, but found no mention of the work's being sold or transferred. In fact, the only documentary records of its existence were a note in Sully's records and a photograph of a small copy of the portrait that the museum acquired as part of a 1984 bequest from another descendant of Benjamin and Maria.
The photograph seemed to be tangible proof that Maria's portrait existed. But at the time of the 1984 bequest, the donor, Fanny Gratz, said the painting seemed to have vanished.
"She said, 'There's supposed to be this Sully portrait but I don't know where it went,' " Guston said.
Where could it have gone? And how could the curator find it?
"What bothered me was that, if I tried to find it, where would I look?" Guston wondered.
The question was not easily answered. There are several branches of the Gratz family, including sizable numbers in Lexington, Ky., where Benjamin and Maria lived. Guston thought about going there and tracking down descendants. But such a time-consuming investigation seemed almost out of the question. And if the painting did not turn up in Lexington, what then?
That's when she came up with the idea to try an Internet matchmaking effort.
"We might not have found it at all," Guston said, "and it showed up in a way much more simple than we would have thought, all because of technology."
Although Roberts had lived with the portrait her entire life, she was delighted at the prospect of reuniting Maria and Benjamin at the Rosenbach. She donated the Sully portrait of Maria, plus the small pastel copy of the painting, and a chair that Benjamin brought from Philadelphia when he settled in Lexington.
Roberts believes the painting of Maria was given to her father by an uncle sometime before 1935, but she is not certain when.
"We've always had the Sully portrait," she said. "It's always been in the house.
"The family started in Philadelphia and I thought it would be nice to give it to the museum in Philadephia."
Guston said Benjamin and Maria are now reunited - Sully painted them as companion portraits - next to each other in the Rosenbach's parlor. It is the first time they have been side by side in at least three-quarters of a century.
"They are up together," said Guston. "Her head is tilted down and his is looking up. They are looking rather affectionately at each other. Our visitors are really enjoying it. Who doesn't enjoy a love story?"
Contact culture writer Stephan Salisbury at 215-854-5594, email@example.com, or follow on Twitter @SPSalisbury.
, followed by reception and art viewing, at the Rosenbach Museum & Library, 2008-10 Delancey Place. Information: www.rosenbach.org or 215-732-1600.