Just as individuals may differ from one another in phenotype because they have different genotypes, because they developed in different environments, or both, relatives may resemble one another more than they resemble other members of the population because they have similar genotypes, because they developed in similar environments, or both. In an experimental situation, we may be able to randomize individuals across environments. Under those circumstances any tendency for relatives to resemble one another more than non-relatives must be due to similarities in their genotypes.
Using this insight, we can develop a statistical technique that allows us to determine how much of the variance among individuals in phenotype is a result of genetic variance and how much is due to environmental variance. Remember, we can only ask about how much of the variability is due to genetic differences, and we can only do so in a particular environment and with a particular set of genotypes, and we can only do it when we randomize genotypes across environments.