Catalyzes the 6-electron reduction of sulfite to sulfide. This is one of several activities required for the biosynthesis of L-cysteine from sulfate. The flavo-protein component catalyzes the electron flow from NADPH -> FAD -> FMN to the hemoprotein component (By similarity). 3 H2O + hydrogen sulfide + 3 NADP(+) = 4 H(+) + 3 NADPH + sulfite Binds 1 FAD per subunit. Binds 1 FMN per subunit. Alpha(8)-beta(8). The alpha component is a flavoprotein, the beta component is a hemoprotein (By similarity).