The adaptive value of polyandry in the absence of direct benefits is often assumed to lie in the production of more viable or more attractive offspring, mediated by additive genetic effects. Alternative models propose nonadditive effects through the selective matching of compatible genomes. If genetic incompatibility, for example, through hybridization, inbreeding, or selfish genetic elements, reduces viability of offspring, selection should favor pre- or postcopulatory mechanisms of inbreeding avoidance. Postcopulatory inbreeding avoidance might be achieved by polyandry in combination with cryptic female choice. Because female spiders have paired and independent sperm storage organs that are only filled one at a time, they have been suggested to be ideal organisms to investigate cryptic female choice. Here we used orb-web spiders of the Mediterranean species Argiope lobata to investigate whether females treat ejaculates from siblings or nonsiblings differently. In double-mating trials using sibling and nonsibling males in all possible combinations, we experimentally manipulated which male mated into which sperm storage organ and subsequently counted spermatozoa in these storage organs. This experimental design allowed us to unambiguously assign ejaculates to individual males. We found no differential storage of sperm from first males but a significantly reduced amount of stored sperm from the second male if he was a sibling. Our results suggest that females cryptically chose sperm to trade up to more compatible males through storing different quantities.