She was born in 1891 in Mińsk as the third child of Tomasz and Eleonora Niżyńskis from Warsaw. They were artists and the only Polish dancers in Łukowicz's wandering troupe with which they toured operettas, musical theatres and circuses of Russian cities and towns. Bronisława was a born dancer – almost literally, as her mother started giving birth while she was dancing a polonaise in Glinka's opera A Life for the Tsar. Bronia didn't have to wait long for success. She was four when she performed with her brothers, Stanisław and Wacław (the latter became famous later on) for the first time, in one of the children's theatres in Nizhny Novgorod. Niżyńska's Early Memoirs read:
We were born as dance artists. We accepted our dancing bodies that we inherited from our parents with no objections. Theatre and dance were a natural way of life since the day we were born. It was just as if the theatre was our natural environment in which everything harmonized with our souls.
Niżyńska was dubbed a ballet child prodigy. In 1900 she started taking dance lessons in the prestigious Imperial Ballet School. One of her teachers was Enrico Cechetti, an Italian mime artist, dancer and author of his own ballet method. In 1908 she started dancing for the Mariinsky Theatre in Saint Petersburg and two years later (together with Wacław) she joined the Ballets Russes group led by impresario Sergei Diaghilev. First she was a dancer in corps de ballet and later – a soloist.
It was there that she met Mikhail Fokine, a remarkable 20th century choreographer. He quickly appreciated Bronisława's distinctive dancing style and aesthetics and cast her in his most famous choreographic works, such as Carnaval, The Firebird and Petrouchka. Wacław also became famous thanks to his roles in Fokine's performances and later became the choreographer's biggest rival. This is how Lucy Moore describes their cooperation in the book Nijinsky: A Life:
When Fokine gave Bronia (editor's note: diminutive for Bronisława) the role of Papillon in his new ballet, Carnaval, he had no time to do more than show her the basic steps, Vaslav (Wacław) helped her create the butterfly's fluttering lightness to match Robert Schumann's prestissimo tempo, working out the placement of the body and the flickering hand movements himself and then helping her to learn them. It was his first piece of choreography for another dancer.
The Niżyńskis siblings inspired and supported each other in their creative work. Bronisława helped her brother with his first choreography, Afternoon of a Faun to Claude Debussy's music, which was premiered in May 1912 in Paris. They worked after hours, with no pianist, and kept it secret from Fokine. Lucy Moore writes that every evening in front of the mirror of Bronisława's dressing table, Wacław arranged his and his sister's bodies in the shape of the Faun and his nymphs. They would spend hours composing one pose or just one movement. Before them, nobody had worked this way. They created something completely new, abstract and dramatically changing the existing approach to music and its relation to choreography.