Answer by taking a quote from the following article:

Heifetz was born into a Russian-Jewish family in Vilna, Lithuania then part of the Russian Empire. His father, Reuven Heifetz, son of Elie, was a local violin teacher and served as the concertmaster of the Vilnius Theatre Orchestra for one season before the theatre closed down. While Jascha was an infant, his father did a series of tests, observing how his son responded to his fiddling. This convinced him that Jascha had great potential, and before Jascha was two years old, his father bought him a small violin, and taught him bowing and simple fingering.

Heifetz married silent motion picture actress Florence Vidor (1895-1977), ex-wife of King Vidor, in 1928, and adopted her daughter, Suzanne. The couple had two more children, Josefa (born 1930) and Robert (1932-2001) before divorcing in 1945. In 1947, Heifetz married Frances Spielberger Spiegelberg (1911-2000), with whom he had another son, Joseph (known as Jay). The second marriage ended in divorce in 1962.  Heifetz's son Jay is a professional photographer. He was formerly head of marketing for the Los Angeles Philharmonic and Hollywood Bowl, and the Chief Financial Officer of Paramount Pictures' Worldwide Video Division. He lives and works in Fremantle, Western Australia. Heifetz's daughter, Josefa Heifetz Byrne, is a lexicographer, the author of the Dictionary of Unusual, Obscure and Preposterous Words.  Heifetz's grandson Danny Heifetz is an accomplished drummer/percussionist. Heifetz's extended family was active in Los Angeles progressive political circles in addition to music and art: they include artist Frances Heifetz-Bloch and her husband Kalman Bloch and daughter Michele Zukovsky--co-principal clarinetists for the Los Angeles Philharmonic--and son Gregory Bloch, violinist for the Italian rock band Premiata Forneria Marconi, It's A Beautiful Day, and member of the Saturday Night Live Band from 1978-80.  Although Heifetz had a "difficult" personality, and has even been described as "misanthropic", he enjoyed the company of selected friends who zealously guarded his privacy, he spoke several languages including flawless English, and was an avid bridge and ping-pong player. His childhood had been difficult; his father was an extremely stern man who, even after Jascha had become the family's sole breadwinner, would still roundly criticise every performance.

Did he have siblings?