  Dirac semi-metal with magnetic atoms as constituents delivers an interesting
platform to investigate the interplay of Fermi surface (FS) topology, electron
correlation, and magnetism. One such family of semi-metal is YbMn$Pn_2$ ($Pn$ =
Sb, Bi), which is being actively studied due to the intertwined spin and charge
degrees of freedom. In this Letter, we investigate the relationship between the
magnetic/crystal structures and FS topology of YbMnSb$_2$ using single crystal
x-ray diffraction, neutron scattering, magnetic susceptibility,
magnetotransport measurement and complimentary DFT calculation. Contrary to
previous reports, the x-ray and neutron diffraction reveal that YbMnSb$_2$
crystallizes in an orthorhombic $Pnma$ structure with notable anti-phase
displacement of the magnetic Mn ions that increases in magnitude upon cooling.
First principles DFT calculation reveals a reduced Brillouin zone and more
anisotropic FS of YbMnSb$_2$ compared to YbMnBi$_2$ as a result of the
orthorhombicity. Moreover, the hole type carrier density drops by two orders of
magnitude as YbMnSb$_2$ orders antiferromagnetically indicating band folding in
magnetic ordered state. In addition, the Landau level fan diagram yields a
non-trivial nature of the SdH quantum oscillation frequency arising from the
Dirac-like Fermi pocket. These results imply that YbMnSb$_2$ is an ideal
platform to explore the interplay of subtle lattice distortion, magnetic order,
and topological transport arising from relativistic quasiparticles.
