Complex sentence: After Nina's death, her diary was found in Soviet archives by Irina Osipova (Memorial), an activist with the human rights organisation Memorial, who was conducting research into opposition to Stalinism and uprisings in the GULAG.
Split into simpler sentences: After Nina's death, her diary was found in Soviet archives by Irina Osipova, an activist with the human rights organisation Memorial. At the time, Osipova was conducting research into opposition to Stalinism and uprisings in the GULAG.