  We consider a tree-level enhancement to the violation of lepton flavour
universality in light meson decays arising from modified $W \ell \nu$ couplings
in the standard model minimally extended by sterile neutrinos. Due to the
presence of additional mixings between the active (left-handed) neutrinos and
the new sterile states, the deviation from unitarity of the leptonic mixing
matrix intervening in charged currents might lead to a tree-level enhancement
of $R_{P} = \Gamma (P \to e \nu) / \Gamma (P \to \mu \nu)$, with $P=K, \pi$. We
illustrate these enhancements in the case of the inverse seesaw model, showing
that one can saturate the current experimental bounds on $\Delta r_{K}$ (and
$\Delta r_{\pi}$), while in agreement with the different experimental and
observational constraints.
