The magnetic phase diagram of Si-doped (0.7 and 1.0 %) ${\mathrm{CuGeO}}_{3}$ single crystals was obtained from an ultrasonic velocity measurement. The magnetic phase diagram is similar to what is generally found in pure spin-Peierls (SP) systems: a SP instability is identified although an additional antiferromagnetic phase is observed at the lowest temperatures. The diagram is weakly anisotropic: a spin-flop (SF) phase is observed for the magnetic field aligned along the c axis and a hard axis along a is deduced from the angular dependence of the SF field. These results are complemented by superconducting quantum interference device magnetization measurements.