Complex sentence: Vladimir von Meck, one of the organizators of the exhibition, and his uncle Nikolai acquired many canvases of Vrubel, including a variant of "Demon", when in 1902 the board of directors of Tretyakov Gallery decided to not buy it.
Split into simpler sentences: Vladimir von Meck, one of the organizers of the exhibition, and his uncle Nikolai acquired many canvases of Vrubel's. This included a variant of "Demon", when in 1902 the board of directors of the Tretyakov Gallery decided to not buy it.