Magnetic shape-memory materials are multiferroics that combine ferroelastic and ferromagnetic order. The diffusionless transformation processes in these materials unroll by the creation of crystallographic domains or twinned microstructures that are coupled to magnetic domain patterns. Miniaturization of the crystallographic domains to the scale of lattice unit cells is afforded by the concept of adaptive modulated martensite. A ferroelastic model for these crystallographic domain patterns is introduced for the exemplary case of the Ni2MnGa Heusler alloy. The pseudo-microscopic model incorporates data from ab initio calculations and phonon dispersion. The twin-boundary energy for the diffuse wall of this model are very low of the order 1 mJ · m−2 fulfilling a key requirement for the formation of adaptive martensite in this material. An extension of the Ginzburg–Landau phase-field model for large strain gradients is introduced to model atomically sharp twin-boundaries.