The standard mountaineering and ski touring approach to South Twin Sister via the south ridge. A moderate objective with a long approach through forest before breaking into open dunite terrain with exceptional views of Baker and the North Cascades.
From the Twin Sisters trailhead on Mosquito Lake Road, follow the trail through forest before breaking onto the open, rocky dunite slopes of the south ridge. The summit of South Twin Sister offers panoramic views of Mount Baker, the Nooksack River valley, and the Cascade Range. The route is a popular ski tour in winter and spring.
Drive south from Bellingham on WA-11, then take Mosquito Lake Road to the trailhead. Road may require high clearance in wet conditions.