The microporous titanosilicate sitinakite, KNa₂ Ti₄ (SiO₄)₂ O₅ (OH)·4H₂ O, was first discovered in the Khibiny alkaline massif. This material is also known as IONSIV IE-911 and is considered as one of the most effective sorbents for Cs⁺ and Sr²⁺ from water solutions. We investigate a mechanism of cooperative crystal chemical adaptation caused by the incorporation of La³⁺ ions into sitinakite structure by the combination of theoretical (geometrical–topological analysis, Voronoi migration map calculation, structural complexity calculation) and empirical methods (PXRD, SCXRD, Raman spectroscopy, scanning electron microscopy). The natural crystals of sitinakite (a = 7.8159(2), c = 12.0167(3) Å) were kept in a 1M solution of La(NO₃)₃ for 24 h. The ordering of La³⁺ cations in the channels of the ion-exchanged form La³⁺ Ti₄ (SiO₄)₂ O₅ (OH)·4H₂ O (a = 11.0339(10), b = 11.0598(8), c = 11.8430(7) Å), results in the symmetry breaking according to the group–subgroup relation P4₂ /mcm → Cmmm.