THE FIRST TIME Charles Robb came to national attention, he took his sister with him. When Robb married President Johnson's daughter, Lynda Bird, his sister Trenny used the position of bridesmaid to launch herself into a New York modeling career.

But this spring, when Robb went around compaigning successfully for nomination as lieutenant governor of Virginia, Trenny was nowhere to be seen. She has had it with instant fame, her mother said, says she has "done her hit with publicity" and has left Frances Robb to explain about her daughter's new life to whoever cares to hear.

"She had to much handed to her on a silver platter - all of a sudden, she was put on top, and she began to ask herself whether she was a good model or if it was all only because her brother married the President's daughter," said their mother. The elder Robbs have moved from Milwaukee to Leesburg. "When everything comes to you at once, some of the glory is taken out of it because you haven't worked for it."

Trenny Robb's silver platter was a modeling contract with the Ford agency in New York. Eileen Ford can hardly recall her now, and gives a nostalgic laugh with the explanation that "so many come and go through here. She disappeared. She just disappeared. I haven't heard her name for years."

Was she any good as a model? "Oh, she was here too brief a time to say."

Trenny's mother said that the "disappearance" was because "she got fed up with the rat race - she had tried it for a while and frankly was not enamored of that New York life" and had met a New York university student named Robert Pforzheimer. They were married in August of 1970, and decided to go off and try farming in New England.

The Pforzheimers are now living on their third farm, near Newport Center, Vt., where they are building a house with their own hands, designed by Trenny Pforzheimer and her brother, David Robb, whose mother describes him as "a perennial student at the University of Wisconsin, who always says that somebody has to provide the comic relief."

The sold their first farm, leased the second and have pastureland and forest land on the one which they occupy.

But the real family business, run by Trenny and Robert Pforzheimer and his twin brother, is called The Stone Factory. There they make pipes - no, said her mother, not plumbing pipes. "You know, it's not illegal to make the pipes: It's what you put in them," she said. "In the 20s, it wasn't illegal to make cocktail shakers."

The Stone Factory also makes "related paraphernalia, and is going into a new sideline of wooden jewelry and hair clips, in heart and bird shapes, designed by Treeny Pforzheimer.

The couple have two children, Trenholm Baylow, 6, who bears two of her mother's family names - "Trenny" is short for Marguerite Trenholm - and is called "Bay" and Jesse Robb, 6.

Trenny is very happy with the farm-and-factory life away from the sportlight, her mother said. "She finds it very satisfying being part of a farm family, building her own house - she is quite an accomplished carpenter - and taking care of her family."

And she's not asking herself any more whether she owes her position to her famous sister-in-law.