Magnetic superlattice peaks are observed in single-crystal neutron-diffraction measurements on orthorhombic ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_{4}$ at reciprocal points of (1/2\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},1/2,0) and (1/2,1/2\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},0) in the tetragonal notation where $\ensuremath{\epsilon}=$0.126$\ifmmode\pm\else\textpm\fi{}$0.003. The La NMR measurement reveals a broadening of the field-swept spectrum below $\ensuremath{\sim}$45 K corresponding to the existence of magnetic order. The remarkable softening of longitudinal sound waves along [110] is observed in the same crystal. The features observed in the neutron diffraction, NMR, and ultrasonic measurements suggest that the dynamical incommensurate spin correlation is pinned by a lattice instability toward the low-temperature tetragonal phase.