I think her maternal grand-mother is supposed to be Jewish. I'm not sure if both of her grand-parents on that side of her family are Jewish. Scarlett has stated that she's celebrated Hanukkah and Christmas. I don't think she's considered Jewish and another person who behaves like Natalie Portman isn't needed.
What did Natalie Portman do ?
Portman was born Natalie Hershlag (Hebrew: נטלי הרשלג) in Jerusalem, Israel. Her father, Avner Hershlag, is an Israeli doctor specializing in fertility and reproduction (reproductive endocrinology). Her mother, Shelley Stevens, is an American homemaker who now works as her agent. Portman's maternal ancestors were Jews from Austria and Russia and her paternal ancestors were Jews who immigrated to Israel from Poland and Romania. Her paternal grandfather's parents died in Auschwitz and her Romanian-born great-grandmother was a spy for the British during World War II.
Portman's parents met at a Jewish student center at Ohio State University where her mother was selling tickets. Her father returned to Israel, but the two corresponded and were married when her mother visited Israel a few years later. In 1984, when Portman was three years old, the family moved from Israel to the United States, where her father pursued his medical training. The family first lived in Washington, D.C., where she attended Charles E. Smith Jewish Day School, but relocated to Connecticut in 1988, and then settled permanently in Long Island, New York, in 1990. Portman has said that although she "really love
the States... my heart's in Jerusalem. That's where I feel at home.She is an only child and very close to her parents, who are often seen with her at her film premieres.
http://en.wikipedia.org/wiki/Natalie_Portman