The Soho Rep had been staging vital productions out of its small Walker Street theater in Tribeca for a quarter century when it was discovered, in 2016, that the company had been in violation of an occupancy limit of 70 people. Through a combination of fund-raising and city agency support, the company has now returned to its home space, which includes a new fire-safety system. It is a fitting improvement, because flames are the essential metaphor of Aleshea Harris’s explosive Is God Is. In this allegorical tale of vengeance and family honor, twin sisters, burned both literally and figuratively, create an inferno that swallows nearly everyone they meet.