I'd go more by the maps and less than the written word, mainly because I've found that a lot of written descriptions were based on maps of the time when they were first written about 20 or more years ago; many streets have been changed in their route, and some in name as well. Also, for example, in Shadowrun, Stapleton Airport is still in use, but in reality it has long since been shut down and much of the land re-zoned for residential (evidence is clear in Google Earth).
About the best I've found for the Sioux/UCAS border is that it starts at the I-70/Colorado Blvd intersection (the north border with the CAS sector) and runs north, until it becomes Vasquez Blvd/Highway 6, intersects with Highway 6/I-76, and follow I-76 all the way up to Hudson (rough estimate where the FRFZ ends and the Sioux nation starts. This puts large features, like DIA and the Rocky Mountain Arsenal National Park, well within the UCAS sector.