Current global constraints in neutrino mixing parameters translate to lax PMNS unitarity relations. In this work, we explore the lack of non-unitarity in the neutrino mixing matrix via the astrophysical neutrino flavor content in both standard paradigms, as well as those involving sterile neutrinos. Astrophysical neutrino flavour composition is a sensitive tool to probe both non-unitarity and the presence of sterile neutrinos over many mass scales, and is a core focus for future high-energy neutrino telescopes. We develop new techniques, including the neutrino flavor tetrahedron, to explore the effects of non-unitary mixing, as well as the significant impact sterile neutrinos have on astrophysical neutrino composition.