Twin Sisters Mountain as a whole is located southwest of Mount Baker, separated from the volcano by the Middle Fork of the Nooksack River.

The Twin Sisters Range is a massive slab of relatively dense rock that was uplifted from major thrust fault activity in the form of a nappe.

The magnesium and iron rich rock was forced upwards from the tectonic collision between the Bell Pass Mélange and the Chilliwack River terrane during the Mesozoic era.

The reddish color of the mountain range may be due to the oxidation of iron and magnesium in the dunite that is exposed to chemical weathering.

Petrology of the Twin Sisters dunite consists of Enstatite rich rocks that contain, along with olivine, significant amounts of chromite, clinopyroxene, and serpentinite.