In the field of photovoltaics, the mixed halide perovskite solar cells (PSCs) have been widely studied due to their excellent power conversion efficiency.[1] However, one of the obstacles of these PSCs is poor stability under ambient conditions such as light, moisture, air, etc. To improve the stability of PSCs, several inorganic cations have been incorporated into the mixed halide perovskite materials, of which Rb and Cs are two of the most effective cations.[2, 3] Nonetheless, the underlying mechanism of how those cations affect the morphology, structure, and composition, and factors affecting the stability of perovskite materials are still unclear and need to be adequately investigated. To achieve this, we conducted electron microscopy studies for Rb and Cs mixed halide perovskite materials. By using focused ion beam (FIB) combined with time of flight secondary ion mass spectrometry (ToF-SIMS), we have demonstrated that Cs+ cations are both segregated at the grain boundary and incorporated into the perovskite structure, while Rb+ cations only form a discrete Rb-rich phase at the grain boundary. We use a combination of low-dose electron microscopy studies and atomic structure simulations to identify a common twinning structure in both cubic and tetragonal phases of the multi-cation mixed-halide perovskite. For the first time, we present clear evidence of {111} twins in the cubic phase and the equivalent {101} twins in the tetragonal phase using selected area electron diffraction (SAED) patterns. We develop a unique way of differentiating between the tetragonal and cubic forms of these twins. The insights presented in this study are essential to promote the research and development of the multi-cation mixed halide perovskite materials.