SNP-based kinship analysis in the beetle Lethrus apterus using single nucleotide polymorphisms (SNPs) as markers demonstrates its complex mating system. Using low-coverage whole-genome sequencing and sibship reconstruction, this shows that half-sibling relationships are more common than full-sibling relationships, indicating non-monogamous mating behaviour. The detection of related offspring over two years suggests recurrent reproduction by the same individuals and considerable male dispersal.