On Tuesday, visitors to the Rosenbach Museum and Library will see what museum officials call their most important acquisitions in many years - a Thomas Sully portrait of Rebecca Gratz, Philadelphia’s renowned 19th-century Jewish American educator, philanthropist and social activist, and a portrait of her brother, Joseph Gratz, by G.P.A. Healy.

The Rosenbach now has two of Sully’s three portraits of Rebecca Gratz, as well as portraits of other Gratz family members, and major holdings of family papers and artifacts.

“We are very excited,” said museum director Derick Dreher. “It’s the most significant purchase in my 12 years as director, both in terms of dollars paid and the intrinsic value of the work to audiences in Philadelphia.”

Dreher declined to disclose the price of the paintings, which were acquired from a Canadian member of the far-flung Gratz clan. He said donors who wish to remain anonymous helped the museum complete the transaction.

The portraits join the Rosenbach’s already considerable holdings of Gratz family materials, including other paintings, decorative furniture, personal accessories, books, and manuscripts.

Several of those items, including another Thomas Sully portrait of Rebecca Gratz, are currently on loan to the new National Museum of American Jewish History on Independence Mall, which opens to the public on Nov. 26.

The latest acquisition, said Judith Guston, the Rosenbach’s curator and director of collections, just happened to come to fruition around the time of the museum’s opening; details of the loan had been under discussion since spring.

The paintings were acquired from Henry Walkem Joseph, a Canadian descendant of Sara Gratz Moses Joseph, niece of Rebecca Gratz.

“My family and I believe this is the right time for the portraits of Rebecca and Joseph to return to Philadelphia,” where they were painted, he said in a statement. “Our hope is that the public will be able to enjoy and learn about these great portraits and understand the contributions this early American Jewish family made to Philadelphia’s history.”

Rebecca Gratz, in particular, is well known for her philanthropy and activism. She was born in 1781 in Lancaster, one of 12 children. Over the course of her long and extremely active life - she died in 1869 - she founded the Female Association for the Relief of Women and Children in Reduced Circumstances, helped establish the Philadelphia Orphan Asylum and began a Hebrew School there, founded the Female Hebrew Benevolent Society, and pushed for a number of other organizations aimed at helping and educating immigrants and the poor. She never married.

Joseph Gratz, her younger brother, a merchant, fought with the First City Troop during the War of 1812, and went on to help found the Philadelphia Institute for the Deaf and Dumb and serve as secretary of Congregation Mikveh Israel, the first Jewish congregation in Philadelphia and one of the oldest synagogues in the country.

The painting of Rebecca dates from 1831, Guston said, and is one of three Sully painted of her in 1830 and 1831; the second is the one on loan to the American Jewish history museum, and the third is in the collection of the Delaware Museum of Art. (Precisely when Healy painted the portrait of Joseph is not known.)

By acquiring the portraits, the Rosenbach now is home to the complete suite of Gratz family portraits that once hung in the dining room of Henry Joseph, the son of Sara Gratz Moses Joseph and Jacob Henry Joseph. The Rosenbach acquired the other paintings in the ensemble as gifts several years ago. These include a Sully portrait of Michael Gratz, Rebecca’s father, and Gilbert Stuart portraits of a sister, Rachel Gratz Moses, and her husband, Solomon Moses.

Henry Walkem Joseph remembers seeing these paintings hung together in his grandfather’s Montreal dining room.

The Rosenbach has been acquiring Gratz family materials for many years. A.S.W. and Philip Rosenbach, the Philadelphia collectors and dealers who founded the museum and library, considered themselves Gratz family descendants. Not surprisingly, kinship feelings led to Gratz-related collecting and has made the museum a mecca for those interested in the family.

“There are a lot of Gratz ‘trekkies,’ ” said Dreher. “This is a family whose name is still a household word in this city.”