Conversation so far:
What was Vaslav Nijinsky biography?
Vaslav Nijinsky was born in 1889 or 1890 in Kiev, Russian Empire, as Wacław Niżyński, to ethnic Polish parents
Who were Vaslav Nijinsky parents?
touring dancers Tomasz Niżyński and Eleonora Bereda
How did Vaslav Nijinsky get interested in ballet?
Vaslav Nijinsky was introduced to dance by his parents
How long did Vaslav Nijinsky perform with his parents?
How long did Vaslav Nijinsky stay in Kiev?
In 1897 Vaslav Nijinsky, his mother and his siblings moved to 20 Mokhovaya Street in St Petersburg
How long did Vaslav Nijinsky  perform with Setov after moving to St Petersburg?
After Josef Setov died about 1894, the Setov Opera company disbanded
What else did Vaslav Nijinsky do?
Vaslav Nijinsky and another boy received training from their father and appeared in an amateur Hopak production in Odessa in 1894
Who was the other boy besidses Vaslav Nijinsky who was trained by their father and appeared in an amateur Hopak production in Odessa in 1894?
Vaslav Nijinsky's brother Stanislav Nijinsky

Question: Who was the third child?
Self-contained rewrite: Who was the third child besides Vaslav Nijinsky and hisStanislav Nijinsky ?