The North Sister is oldest, with towering rock pinnacles and brilliant glaciers. It has not erupted since the late Pleistocene.
The Middle Sister is the smallest and most poorly studied. It is the second youngest. No reports on its potential for eruptions can be made.
The South Sister is the youngest volcano of the trio. It last erupted about 1600 years ago. It has a well developed crater. It is the most dangerous, with fresh-looking pumice all around it.
In 2000, a satellite discovered that there was a deforming uplift near the South Sister. This could mean that she is awakening. Depending on the type, the eruption could either result in quiet emissions of lava, of the town being buried, or even a total levelling of the mountain.
The Sisters were named Faith, Hope, and Charity by early settlers.