Offspring and kin care are common in nature, while non-kin societies are unusual due to their susceptibility to cheaters. Here, we investigated the kinship of mast-building amphipods, Dyopedos bispinis (Gurjanova, 1930). Our goal was to determine if all mast inhabitants are descendants of a single founder female or if they represent a more complex social structure. We sequenced and assembled the complete mitochondrial genome of D. bispinis along with 59 partial genomes from four masts. One of the studied masts contained several adult females with embryos, all of which had identical partial mitochondrial genome sequences. This shows that masts can be inhabited by individuals from different generations. Mitochondrial genome sequences of ten mother-embryo pairs confirm maternal mtDNA inheritance in D. bispinis. However, another mast contained several groups of female individuals exhibiting pronounced (~0.7 substitutions per 1000 b.p.) distance between the groups. The genetic distance between groups from the same mast was not less than the genetic distance from specimens of other masts. This suggests collective usage of the mast by non-related families. Given that adult female D. bispinis individuals invest resources into mast maintenance, this case implies non-kin cooperation among amphipods. Overall, our study provides an insight into the family structures of mast-inhabiting amphipods and presents a new model for studying the cooperation of non-related individuals.