I don't believe this is true.My Little Green Book on "The Genealogy of Eliza Ann Allman" page 6, states that re:George Allman, "...He married a Miss Phebe Otis born in 1796 and died in 1824 [of cholera according to my research and her Ohio mortality record].Six children were born to them, viz:four sons and one daughter were born to them.All but the youngest lived to mature years. ...I knew nothing of Aunt Phebe.Her body lies in the family burying ground beside George Washington [I believe this refers to George Washington Allman].Nothing marks the exact spot."President George Washington had no niece named Phebe.
On page 7 of above book, it says, "Aunt Phoebe had left 6 children, and 9 more were added to the family after Uncle George and Aunt Maria [George's second wife] were married.She now lies beside Uncle George in the Loudenville, Ohio,cemetery."I take this to refer to Phebe Otis Allman.
I am descended from Phebe and George Allman them through Daniel Allman > Eliza Ann Allman Brown > to Mary Lisetta Brown (my great-grandmother) > Grant Walker, my grandfather.