Twin Sisters Peaks, a pair of mountains near Estes Park on the boundary between Rocky Mountain National Park and Roosevelt National Forest, form part of the park’s eastern edge and have long been a popular hiking destination. Once known locally as Lily Mountain, they received their official present name in 1908 after Ellsworth Bethel submitted it in 1907. A summit trail was first built in 1914, along with a Forest Service fire lookout operated for five summers by Swiss mountaineer Walter Kiener, whose presence helped attract visitors. The trail was rebuilt in 1936, and a 2013 mudslide left lasting visible damage on the mountain’s western side.