Oregon's Three Sisters, with a view looking northward. The closest in the photo is South Sister, which is the tallest of the three stratovolcanic peaks.
Most recent eruption estimated at 1,450 to 1700 years ago
Oregon’s Three Sisters are a complex volcano, meaning they are all part of one volcano. They are each stratovolcanoes sitting atop old shield volcano structures, and they are aligned north-south in close proximity to one another. Called South Sister, Middle Sister, and North Sister, these are the third, fourth, and fifth highest peaks in the state of Oregon, trailing only Mount Hood and Mount Jefferson. Each tops 10,000 feet (3,000 meters) elevation.
The three peaks collectively have 15 glaciers, which is almost half of the 35 glaciers found in Oregon’s portion of the Cascade Mountain Range. Each was very active with eruptions from the main peaks occurring during the Pleistocene Ice Ages, although now the only main peak that is thought to still be a threat for eruptions is South Sister. USGS has rated the threat level as “high.”
The volcanoes sit in a wilderness area, called the Three Sisters Wilderness. Popular recreational activities include hiking, camping, rock climbing, and fishing. The southern two volcanoes are considered moderately difficult to climb, although North Sister is regarded as very difficult and requires mountain-climbing expertise and specialized equipment.
In this article I use basic volcanology terms including stratovolcano, shield volcano, and others. For definitions, see my article Types of Volcanoes and Basic Volcanology Terms.
North Sister is very heavily eroded in comparison with the other two peaks. The main peak is extinct, and old magma that had hardened within the volcano has become exposed due to the erosion.
The most recent eruptions from the main peaks
The main peak of North Sister last erupted about 120,000 years ago. It is considered to be extinct, and has been heavily eroded by glaciers, with the central plug of hardened magma becoming exposed.
Middle Sister’s and South Sister’s main peaks last erupted about 14,000 years ago, with South Sister having erupted slightly more recently. Middle Sister has since become eroded, although not to the extent of the northernmost peak. South Sister however, unlike the other peaks, has an uneroded summit crater that is about ¼ mile (0.4 km) in diameter, and it holds a small crater lake called Teardrop Pool, which is the highest lake in Oregon.
Middle Sister during winter and covered with snow. The main peak is likely extinct, or at least dormant and inactive.
The most recent eruptions from associated vents
Although North Sister’s main peak is extinct, an eruption from a vent on its northwest slope occurred somewhere between 1,450 and 1,700 years ago. This is the most recent eruption from the Three Sisters volcanoes.
South Sister had an eruption from a vent on its southern slope about 2,050 years ago. Although this may sound like a long time ago, it’s very short in geological terms, and active volcanoes are typically defined as having erupted in the past 10,000 years and having potential for further eruptive events. 
The South Sister stratovolcano as seen from the southeast. The mountain is closely monitored due to high risk of another eruption occurring from its main peak.
South Sister potential for eruption
As stated above, South Sister is considered to be the most active of the three peaks, and is at high risk of another eruption from its main peak. If an eruption were to take place, the city of Bend, Oregon, which is 20 miles (32 km) to the east, could find itself under a layer of volcanic ash an inch (2.5 cm) or more deep. This would cause a lot of problems, and would be very costly to clean up.
South Sister, which is the tallest of the three peaks at an elevation of 10,358 ft (3,157 meters), is closely monitored. In 1997 a period began in which the western slope rose 11 inches (28 cm). A notable earthquake swarm occurred in 2004. These activities appeared to cease in 2007, although in 2013 it was confirmed that uplift is still occurring at a reduced rate, and consequently monitoring of the volcano has further increased.
The breathtaking view from atop South Sister
The video begins with a view to the north of the two other volcanic peaks, and some other notable Cascades volcanoes can be seen further away in the distance. The view then rotates to the east.