Mount Baker is a conical volcano composed of igneous and metamorphic rocks. Mount Shuksan was once seafloor material that was sucked down under the Earth's surface until it got so hot the rock melted into Shuksan Greenschist and rose up to the surface like an ice cube in water. The Twin Sisters formed even deeper in the mantle of the Earth and reached the surface by complex plate tectonics. The Twin Sisters massif is a blob of dunite, composed mostly of olivine and is very resistant to erosion, which accounts for its sharp pinnacles compared to the softer domed summit of Mount Baker. The Mount Shuksan massif is also very resistant to erosion.
Active glaciers cling to their summits.
This is only a rudimentary explanation of the complex processes occurring in the Nooksack drainage. If readers are interested in learning more about local geology, Rowland Tabor and Ralph Haugerud explain it all in Geology of the North Cascades. The book is easy to understand with lots of visual aids. It contains great geological summaries of popular destinations in the North Cascades. The book is available in local bookstores.