Two ways to test
1. A consumer DNA kit for each of you. You both take a test like AncestryDNA or 23andMe. If you used the same company, you'll appear in each other's match list with a shared cM number. If you used different companies, download both raw files and compare them directly.
2. A lab sibling (kinship) test. You both swab, a lab compares your DNA at specific markers, and you get a report with a "siblingship index" or probability. Lab tests are usually more expensive per person than a sale-priced consumer kit, and legal versions cost more again.
Full vs. half siblings: what the numbers say
| Relationship | Average shared cM | Typical range |
|---|---|---|
| Full siblings | 2,613 | 1,613–3,488 |
| Half siblings | 1,759 | 1,160–2,436 |
| Grandparent, aunt/uncle | ~1,750 | similar to half siblings |
| First cousins | 866 | 396–1,397 |
The ranges overlap, so the total alone isn't always enough. The tie-breaker is fully identical regions: stretches where you match on both copies of a chromosome. Full siblings inherit from the same two parents, so about a quarter of their DNA is fully identical. Half siblings have almost none. 23andMe shows this directly. Other tools can calculate it from raw data files.
Half sibling, grandparent or aunt?
All three share about 1,750 cM, so the DNA total can't tell them apart. Ages usually can. Beyond that, paternal half-brothers share a Y-DNA line, and maternal half-siblings share mitochondrial DNA, which some tests report.
Which test should you pick?
| Your situation | Best option |
|---|---|
| Curious, or checking a hunch | Consumer kit for each of you |
| Donor-conceived or adopted, searching for siblings | Consumer kit (the bigger the database, the better) plus donor sibling registries |
| Immigration, inheritance or court | Legal sibling test from an accredited lab, with chain of custody |
| One of you already tested | The other takes the same brand, or you compare raw files |
For most people, a consumer kit is the better value: it answers the sibling question and finds every other relative who has tested. Here's how the main kits compare.
How accurate are sibling tests?
Consumer kits are very reliable at telling full siblings from strangers, and the cM number plus fully identical regions usually settles full vs. half. Lab sibling tests are less conclusive than paternity tests, because siblings don't share a fixed amount of DNA. Including a known parent's sample makes them much stronger.
When it really matters: dating
Some sperm donors have dozens, even hundreds, of offspring, and the US sets no legal limit. If you're donor-conceived or adopted, a quick DNA comparison before things get serious isn't paranoid. It's practical. That's exactly what our relatedness check is built for.