South Twin is the highest point on the Twin Sisters Range west of Mount Baker. The lower North Twin is probably climbed more frequently. Routes on South Twin involve a complex approach on logging roads and faint use trails, and extended scrambling up long ridges.
The USGS map for South Twin shows a 7000-foot contour at the west end of the summit ridge, however the Sisters Benchmark about 500 feet ESE is actually the high point, even though it does not have a 7000 foot contour. Adding to the mystery, the benchmark is at 6932 feet, which seems very low given the contours on the map.