Sibling DNA Testing: Full, Half, or Unrelated
Published January 29, 2026 · Medically reviewed by the partner laboratory's medical director, AABB-accredited relationship-testing laboratory
A sibling DNA test analyzes 34+ STR markers between two people to calculate the probability they share zero (unrelated), one (half-siblings), or two (full siblings) biological parents. Results are usually stated as a Sibling Index and interpreted alongside known parent information for maximum certainty.
How it works
Because siblings share, on average, 50% of their DNA (full) or 25% (half), the pattern of shared markers gives a statistical probability of biological relationship. Adding a known parent's DNA (usually the shared mother) dramatically increases certainty.
Ready to move forward? Order our $99 at-home test — AABB-accredited, results in 1–2 business days.
Full vs half sibling
Full siblings share both biological parents. Half siblings share one. The test reports which is most probable — but a definitive answer often requires the shared parent's sample as a reference.
When to use it
When paternity testing isn't possible because the father is unavailable, sibling testing can confirm or rule out biological relationship — useful for inheritance disputes, Social Security claims, family reunification, and personal knowledge.