Closely-related species coexisting in sympatry provide critical insight into the mechanisms underlying speciation and the maintenance of genetic divergence. Selfing may promote reproductive isolation by facilitating local adaptation, causing reduced hybrid fitness in parental environments. Here, we propose a novel mechanism by which selfing can further impair interspecific gene flow: selfing may act to ensure that non-hybrid progeny systematically co-occur whenever hybrid genotypes are produced. Under a competition arena, the fitness differentials between non-hybrid and hybrid progeny are then magnified, preventing development of interspecific hybrids. We investigate whether this “sibling competition arena” can explain the coexistence in sympatry of closely-related species of the plant fungal pathogens (Microbotryum) causing anther-smut disease. The probabilities of intra-promycelial mating (automixis), outcrossing, and sibling competition were manipulated in artificial inoculations to evaluate their contribution to reproductive isolation. We report that both intra-promycelial selfing and sibling competition significantly reduced rates of hybrid infection beyond that expected based solely upon selfing rates and non-competitive fitness differentials between hybrid and non-hybrid progeny. Our results thus suggest that selfing and a sibling competition arena can combine to constitute a barrier to gene flow and diminish selection for additional barriers to gene flow in sympatry.