Resource allocation to offspring is the battleground for various intra-familial conflicts. Understanding these conflicts requires knowledge of how the different actors (mother, siblings with different paternal genotypes) influence resource allocation. In angiosperms, resource to seeds is allocated post-fertilization and the paternally inherited genome in offspring can therefore influence resource allocation. However, the precise mode of resource allocation, and in particular the occurrence of sibling rivalry, has been rarely investigated in plants. In this paper, we develop a new method to analyze the resource allocation traits of the different actors (maternal sporophyte and half-sibs) using the data obtained from a large-scale diallel cross experiment in maize involving mixed hand pollination and colors markers to assess seed weight of known paternity. We found strong evidence for the occurrence of sibling rivalry: all resource to ears was allocated competitively and offspring with different paternal genotypes aggressively competed for this resource, entailing a measurable direct cost. We also show how resource allocation can be described for each genotype into two maternal (source effect, average sink responsiveness) and two offspring traits (ability to attract maternal resource, competitive ability towards siblings). We will discuss how these findings help understanding how genetic conflicts shape resource allocation traits in angiosperms.