Sibling DNA testing is designed to determine sibling genetic relationships. Such tests establish the statistical probability of whether the siblings share the same two parents. A DNA sibling test can be performed to analyze the relationship of biological siblings by collecting samples from both siblings; however, including one common parent increases the accuracy of this type of analysis, and is highly recommended.

While conducting sibling DNA testing, the DNA laboratory determines the genetic profile of the alleged siblings and, based on the type of genetic material inherited by each sibling, a DNA index is determined. Based on the results of the DNA index, the sibling relationship is established and a higher value DNA index indicates a greater likelihood that the two individuals are true biological siblings. For instance, a DNA index of less than 1.00 indicates non-relatedness, whereas a DNA index of greater than 1.00 indicates that the two tested individuals are more likely to be true biological siblings.

A half-sibling DNA index can also be obtained to determine whether the siblings share only one common parent, versus two common parents. The ratio of the half DNA index versus the full DNA index establishes whether the two individuals share one parent or two parents. DNA testing can also help determine if twin siblings are fraternal or identical. For instance, if the DNA profiles of the twins are different then the siblings are considered fraternal; however, if the profiles are identical then a probability of being identical twins is calculated.

Sibling DNA tests are unlike paternity tests and do not provide a conclusive result; however, the tests might provide you with an indication of whether you are more likely to be or not to be the true biological sibling of the other tested individual.