As they have been doing off and on for almost half a century, Yehudi Menuhin and his sister, Hephzibah, joined forces at the Kennedy Center last night for a handsome program of violin and piano music. At this point their partnership seems effortless, more a matter of instinct than conscious thought.

The concert included two of the loveliest works for piano and violin, Brahm's Second Sonata in A major and Franck's Sonata, also in A major. Both received a particularly warm and expansive treatment with flowing lines from Yehudi and fluid passage-work from Hephzibah.

If Yehudi Menuhin's playing lacked some of the tauntness and precision of past performances, the loss was counter-balanced by his capacity to touch upon universal qualities in the music. Spinning out the lines in Bach's Third Partita for solo violin he conveyed a sense of timelessness that echoed the freedom of Eastern cultures. He also projected with particular affection the Romanian Folk Dances of Bartok, with whom he was closely associated in the composer's final years.

Though less romantic in her approach than her brother, Hephzibah bent easily with his style while laying down a firm rhythmic base. Both artists contributed their fees for the evening to the World Service Authority, a D.C.-based human rights organization. Yehudi Menuhin has been a WSA World Passport holder and registered World Citizen since 1954.