Multiscale twinning microstructure of single crystals of Ni-Mn-Ga alloys is of great interest. The highest value of magnetic field induced strain (up to 6% elongation) is observed in the compositions with such kind of microstructure. This phenomenon is widely investigated by neutron and x-ray powder diffraction. However, the models which are used in this studies are approximations of modulated crystals and they do not take into account stacking faults. In current investigation we propose to study single crystal diffuse neutron scattering and use a model which takes into account probability of existence of stacking faults and a multiscale twin hierarchy.