  We derive the effective action for the collective spin modes in iron-based
superconductors. We show that, due to the orbital-selective nature of spin
fluctuations, the magnetic and nematic instabilities are controlled by the
degrees of orbital nesting between electron and hole pockets. Within a
prototypical three-pockets model the hole-electron orbital mismatch is found to
boost spin-nematic order. This explains the enhancement of the nematic order in
FeSe as compared to 122 compounds, and its suppression under pressure, where
the emergence of the second hole pocket compensates the orbital mismatch of the
three-pockets configuration.
