The most competitive men's and women's races in the 18-year history of the National Capital Open became a family affair yesterday. Olympic silver medalist Rebecca Twigg won the featured women's race and an hour later, her husband of six months, Mark Whitehead, won the featured men's race, defeating a field of the United States' top cyclists.

For more than 4,000 spectators, it was a wild and windy day on the Ellipse, with high-speed crashes sending a dozen racers to the hospital and knocking about 50 of the more than 500 racers out of their events. None of the injuries, which ranged from minor scrapes to a dislocated shoulder, were considered serious. All racers wore hard helmets under new rules of the U.S. Cycling Federation.

Whitehead, a member of the 1984 U.S. Olympic team who was second in the 1983 National Capital Open, was unseeded but has recently won several criterium races around the country -- short loop races such as the Open. He won the 60-kilometer (36-mile) race in a final mass sprint of more than 100 out of the 137 racers who started the men's top amateur-professional race.

Bruce Donaghy, who has won the Open three times (plus twice as a junior), was a wheel behind Whitehead. Davis Phinney, who just won the week-long Beatrice Bicycle Classic in Texas and is considered one of the world's best criterium riders, was third. Olympic bronze medalist Leonard Harvey Nitz, who won here last year, was caught in the middle of the tightly bunched pack at the finish.

Whitehead said he was behind Phinney in the final 200 yards "when Phinney topped out, going as fast as he could go and I felt good and went by . . . though Donaghy almost caught me. I was just surprised there wan't a crash on that last lap."

Twigg won by several bike lengths against a field of many of the best women sprinters in another mass finish -- there were no successful breakaways in any of the nine races on the flat, cornerless oval. Twigg, who won here in a rare breakaway in 1982, said, "This is a big race, a classic, although there are not many foreign riders" who enter it. Sally Zack, an unheralded rider from Michigan, was second. Betsy Davis, who has won here four times, was third.

This year's Open, sponsored by its founders, the Embassy of France and Mel Pinto Imports, offered $11,000 in prizes plus "primes" -- lap prizes such as a dozen bagels, fingerless bike gloves and massages.

Most of this year's Open riders are members of national cycling teams, with many here because the Open is now one of 26 races in the $80,000 7-Eleven Cup Series.

In other races yesterday, Bobby Phillips of Baltimore and Jim Montgomery of Herndon finished one-two in the Veterans race for 35- to 40-year-olds and Ramsey Stuart of College Park won the junior 9-11 division. Gregory Morin of Annapolis was second in men's Senior IV and Angel Grabill of Chevy Chase was second in women's B.

Other winners were: Kyou Min of Dobbs Ferry, N.Y., (men's Senior III); Tim Norris of Rocky Mountain, Ohio, (men's Senior IV); Jessica Grieco of Emerson, N.J., (women's B); George Hincapie of Farmingdale, N.Y., (junior 12-13) and John Coyle of Detroit (junior 14-17).