I think you're right that this is in Sciaridae. I've been told the shape of that Y-shaped vein in the wing is distinctive for the two families of fungus gnats (Sciaridae [dark-winged fungus gnats] and Mycetophilidae [fungus gnats]) - at least compared to other gnat or midge-like families. According to Borror, Triplehorn, and Johnson (1) a distinctive venation character for Sciaridae is that the two veins near the leading edge of the wing are both straight, as can be seen in your photo. There are probably more general differences between the families that I'm not aware of, but your photo shows the veins very clearly.