  We analyse the $\overline B_s^0\to K^+l^-\bar \nu$ and $\overline B_s^0\to
K^{*+}(\to K\pi)
  \ell^-\bar \nu$ decays that are valuable for extracting the CKM matrix
element $|V_{ub}|$. We calculate the differential and integrated partial widths
in units of $|V_{ub}|^2$ based on various calculations of hadronic form factors
and in particular the latest Lattice QCD calculation of the $B_s\to K^*$ form
factors. For the decay $\overline B_s^0 \to K\pi \ell\bar \nu$, we formulate
the general angular distributions with the inclusion of the various
partial-wave $K\pi$ contributions. Using the results for the $K\pi$ scalar form
factor calculated from unitarized chiral perturbation theory, we explore the
S-wave effects on angular distribution variables and demonstrate that they may
not be negligible, considering the high precision expected in future
measurements. We also briefly discuss the impact of the S-wave $\pi\pi$
contributions in the $B^-\to \pi^+\pi^-\ell \bar \nu$ decay and provide
estimates for the mode $B^-\to K^+K^-\ell \bar \nu$. The studies of these
channels in future can not only be used to determine $|V_{ub}|$, but may also
provide valuable information on the $K\pi$ and $\pi\pi$ phase shifts.
