  Here we report the evolution of structural, magnetic and transport properties
in MnBi$_{2-x}$Sb$_x$Te$_4$ (0$\leq x \leq$2) single crystals. MnSb$_2$Te$_4$,
isostructural to MnBi$_2$Te$_4$, has the lattice parameters of
\textit{a}=4.2445(3)$\AA$ and \textit{c}=40.869(5)$\AA$, respectively. With
increasing Sb content in MnBi$_{2-x}$Sb$_x$Te$_4$, the \textit{a}-lattice
decreases linearly following the Vegards law while the \textit{c}-lattice shows
little compositional dependence. The \textit{a}-lattice contraction occurs by
reducing Mn-Te-Mn bond angle while Mn-Te bond length remains nearly constant.
The anisotropic magnetic properties suggest an antiferromagnetic order below
T$_N$=19\,K for MnSb$_2$Te$_4$ with the magnetic moments aligned along the
crystallographic \textit{c}-axis. The antiferromagnetic ordering temperature
slightly decreases from 24\,K for MnBi$_2$Te$_4$ to 19\,K for MnSb$_2$Te$_4$.
More dramatic change was observed for the critical magnetic fields required for
the spin-flop transition and moment saturation. With increasing Sb content,
both critical fields decrease and in MnSb$_2$Te$_4$ a small field of 3\,kOe is
enough to saturate the moment. In high magnetic fields, the saturation moment
shows significant suppression from 3.56$\mu_B$/Mn for MnBi$_2$Te$_4$ to
1.57$\mu_B$/Mn for MnSb$_2$Te$_4$. Data analyses suggest that both the
interlayer magnetic interaction and single ion anisotropy decrease with
increasing Sb content. The partial substitution of Bi by Sb also dramatically
affects the transport properties. A crossover from n-type to p-type conducting
behavior is observed around x=0.63. Our results show close correlation between
structural, magnetic and transport properties in MnBi$_{2-x}$Sb$_x$Te$_4$ and
that partial substitution of Bi by Sb is an effective approach to fine tuning
both the magnetism and transport properties of MnBi$_{2-x}$Sb$_x$Te$_4$.
