'South Sister' is the unofficial name for the 11,376 foot southern summit of the prominent Twin Sisters Mountain. Somewhat confusingly, Gerry Roach lists this peak on his RMNP Peak List as "Twin Sisters Mtn"; however, the highest point of Twin Sisters Mountain is East Twin Sisters Peak, 0.4 miles to the north.
To be clear, this page is for the 11,376 foot south summit of Twin Sisters Mountain, not the 11,413 foot and 11,428 foot summits (given the name Twin Sisters Peaks) to the north. Due to the 356 feet of prominence between this peak and East Twin Sisters Peak, this summit has a hard ranking of its own, independent of the Twin Sisters Peaks. This goes back to the discussion of whether a summit is a ranked peak because it has a name or because it has at least 300 feet of prominence. Another rule that some people apply is the requirement of 0.5 miles or more of horizontal distance between a peak and its parent. 'South Sister' would not be a ranked peak if this distance rule was applied. However, when using the "300 foot prominence rule" as the sole definition of a ranked peak (which is the sole rule that Gerry Roach uses for his peak list), this peak is the 69th highest point in the park, East Twin Sisters Peak is the 67th highest point in the park, and West Twin Sisters Peak is unranked because there is only 73 feet of prominence between it and East Twin Sisters Peak. All peak lists are fairly arbitrary and not everybody goes by Gerry Roach's list.
The "twin" aspect of Twin Sisters Mountain applies to East Twin Sisters Peak and 'South Sister' when the mountain is viewed from the east, not the East and West Twin Sisters Peaks that are reached by the Twin Sisters Trail.
'South Sister' resides on the east side of the Tahosa Valley, east of Longs Peak. The mountain resides partially in an island of Rocky Mountain National Park separate from the main portion of the park, and partially in Roosevelt National Forest.
A popular trail leads to the summits of the nearby Twin Sisters Peaks, and dozens of people ascend those peaks every day in the summer. There is no trail to the summit of 'South Sister', however. As a result, the peak sees little traffic despite the throngs of people scrambling over the peaks just to the north.
The summit provides incredible views of the massive glacial cirque formed by Mount Lady Washington, the East Face of Longs Peak, and centennial thirteener Mount Meeker.