Sibling DNA Testing
Ideally, testing their parents' DNA is the most accurate way to determine if two or more individuals are full or half-brothers, sisters or any combination. However, one or both parents may be unavailable or unwilling to be tested. In that case, the possible siblings themselves can undergo testing to some degree of reliability.

Sibling - or kinship - DNA testing kits are available from online labs or testing can be done with the assistance of a health care professional or genetic counselor at an actual lab. Either way, the simplest test is the buccal swab test, where the inside of the cheek is swept for saliva samples which are then tested by a lab to establish a DNA profile for each test participant. The results of the separate tests are compared and the likelihood of relatedness can be predicted by using a "sibling index" of numerical values. Likewise, a relationship can be predicted with a half-sibling test that determines if the individuals share just one parent. As a rule, siblings who share two parents will have about 50 percent of their DNA in common, and half-siblings will have about 25 percent of their DNA in common.

If the sibling index produces a score of less than 1.00, relatedness is not likely. The higher the score over 1.00, the greater the chances the individuals are siblings. Testing at least one shared parent can lead to greater accuracy in the test results and is highly recommended whenever possible. Unlike paternity testing, conclusive results are not possible but accuracy is improved when maximal DNA information is provided.

Sibling DNA testing can also indicate whether two siblings are twins, either identical, which may not be visually obvious after many years, or fraternal.

Not all online labs offer sibling testing. When you find those that do, look for members of the American Association of Blood Banks, the accreditation agency that sets exacting standards for member testing labs to assure you of reliable results.