By Patricia Reaney

NEW YORK (Reuters) - Few people have had more influence on the impact of avant-garde art in early 20th century Paris and the careers of Pablo Picasso and Henri Matisse than the writer and collector Gertrude Stein and her siblings.

In a new exhibition, "The Steins Collect: Matisse, Picasso, and the Parisian Avant-Garde" which opens at the Museum of Modern Art on Tuesday and runs through June 3, their patronage and friendships with the emerging artists of their day is chronicled through the works they collected.

"During the first decade of the 20th century arguably the Steins did more than any other collector or dealer to advance the cause of modern art," said Rebecca Rabinow, curator in the museum's Department of Modern and Contemporary Art.

"They were among the very few who could appreciate Matisse and Picasso early on when these artists were still relatively unknown."

The bulk of the 200 paintings, sculptures and works on paper featured in the exhibit were once owned by the Steins -- Gertrude, her brothers Leo and Michael and his wife Sarah. Many hung on the walls of their apartment at 27 rue de Fleurus in Paris, which is recreated in the exhibition.

One painting, Matisse's "Woman with a Hat" is the centerpiece and inspiration for the show and was owned independently by each of them. It's heirs had stipulated that it can leave its home at the San Francisco Museum of Modern Art only once, which was the impetus for the exhibition.

"It was really because this is a once-in-a-lifetime opportunity for that picture to travel," Rabinow said about the exhibition which was eight years in the making.

Unlike previous American collectors who were wealthy and purchased works by contemporary artists and took them back to the United States, the Steins were an upper-middle-class Jewish family who lived on income from investments and rental properties in San Francisco.

They lived in Paris, befriended the artists, and made their collections accessible through their weekly Saturday salons at their apartments.

After moving to Paris in 1903, Leo Stein began collecting works by unknown artists. Works Picasso and Matisse were among his first purchases. His siblings followed and they pooled their resources to collect paintings by Cezanne, Gauguin, Renoir, Pierre Bonnard, Maurice Denis, Juan Gris and other artists.

Through their salons the Steins introduced dealers, collectors and other artists to their collections. They not only had an impact on the dealers of their time and introduced the public to the works of the artists, the Steins also introduced Matisse to Picasso.

"They opened up their homes to an international group of artists and collectors and dealers so for the first time people had an opportunity to come to a safe environment where people weren't laughing and looked at the art on walls," Rabinow explained.

"And it was thanks to the Steins that this aesthetic was spread back to America, throughout Europe, Scandinavia and it would have taken a lot longer to have happened if the Steins weren't around."

When Leo decided to leave Paris in 1913, he and Gertrude divided their collection, with Gertrude taking the Picasso paintings and Leo getting 16 Renoirs. Sarah and Michael's collection consisted of many works by Matisse, among other artists.

From the turn of the century through two World Wars and afterwards the exhibition traces the Stein family link and impact on the leading artists of their time.

"For me, this exhibition provided an 'Alice in Wonderland'-like rabbit hole to slide back down to the first two decades of the 20th century. I think it will give people an opportunity to see a little bit more of the complexities and the relationships, not only of the artists but their patrons, and I hope bring it to life for them," said Rabinow.

(Reporting by Patricia Reaney; editing by Paul Casciato)