In the Altheim graveyard, established in the 5th century by a group of Northern European descent, we inferred a demographic shift in the 6th century with the integration of newcomers with ancestry typical of a nearby Roman military camp, likely as a result of the collapse of Roman state structures. We reconstructed multigenerational pedigrees and, using a novel approach to infer ancestry of unsampled relatives, inferred immediate intermarriage between incoming and local groups, with a distinct tendency for men from former Roman background marrying women of northern descent. Burial proximity correlates strongly with kinship, in some cases spanning six generations. These communities were organized around small family units, exhibited loosely patrilineal or bilateral descent patterns, practiced reproductive monogamy, and avoided close-kin marriages.