If you are seeking to establish that two individuals are biological siblings, the best way is to test their parents. However, if the alleged parents are unavailable or are not willing to provide their DNA sample, a Siblingship Test is performed to analyze the relationship between two individuals
Inclusion of a common parent
If the mother is known, then it is recommended that she should be included in the Siblingship Test as well. Including the mother(s) in a Siblingship Test can increase the possibility of combined likelihood by 15%, from 83% to 98%. If either known parent is involved, the accuracy of this test increases significantly.
Siblingship DNA index
If the calculated siblingship DNA index is less than 1.00., then those two alleged individuals are not at all related. But if the index is more than 1.00, it means that the tested individuals are more likely to be biological siblings. The higher the index value is, the greater the possibility of two alleged individuals being full biological siblings.
Full Siblings vs. Unrelated
In these tests, the DNA of two alleged individuals is compared keeping in mind that they either have the same biological parents or not. Such a test is used so often in US Immigration Cases where an individual, who is an American citizen sponsors an alleged sibling who is applying for an immigrant visa.
Full Siblings vs. Half Siblings
In these tests, the DNA of two individuals having a common biological mother is tested. Their DNA is compared to that of the mother. For this type of Siblingship Tests, we recommend that the DNA sample from the mother should also be tested. The testing of the same biological parent enables us to conclude what traits acquired by the two individuals originate from their father, hence effectively helping us in increasing the conclusiveness of the test.
Half Siblings vs. Unrelated
The DNA of two people with different biological mothers are compared to determine the likeness of both of them having a same biological parent. Again, in this type of test as well, we recommend that samples from both of the biological mothers be tested to correctly find the genes inherited by these individuals from their biological father(s).
The Twin Zygosity
In this test, the DNA of 2 individuals is tested against each other to conclude if they are identical or fraternal twins. This type of siblingship test does not require parents’ DNA to be sampled.