What a half sibling looks like
AncestryDNA usually does not label a match as “Half sibling” on its own. A half sibling typically appears in the Close Family group, and the label may read something like “Close Family” or “Close Family - 1st Cousin”, with a shared DNA amount in the range of roughly 1,300 to 2,000 cM. The exact wording on your screen can differ.
Shared DNA range
Half siblings share one parent, so on average they share about 25% of their DNA. The Shared cM Project, a community dataset of reported matches, gives an average near 1,760 cM with a range from about 1,160 to 2,436 cM. Individual results vary, and those figures are statistical, not exact rules. Ancestry also uses its own estimates, so its range may not line up exactly with the Project’s.
Relationships that overlap
The same amount of DNA can also fit:
- Grandparent and grandchild
- Aunt or uncle and niece or nephew
- Double first cousins
- Sometimes other close relationships
This overlap is why shared DNA alone cannot name the relationship. See how to change a relationship label for how to record what you learn.
How to tell
- Check ages. A match decades older or younger than you may be a grandparent, aunt, uncle, niece or nephew rather than a sibling.
- Use shared matches. A half sibling matches relatives on only one of your parents’ sides. If all common matches are on your mother’s side, the shared parent is probably your mother. See finding your mother’s and father’s sides.
- Check Parent 1 and Parent 2. If SideView has assigned the match to one group, it supports a one-parent connection. Read what Parent 1 and Parent 2 mean.
- Use a relationship probability tool. Enter the shared cM into a tool such as the DNA Painter Shared cM Project tool to see which relationships are plausible.
- Test a parent or other relative. If a parent has tested, a half sibling will link to only one parent, which is very strong evidence.
Full vs half
Full siblings usually share about half their DNA on average, often somewhere around 1,600 to 3,500 cM, with an average near 2,600 cM, and typically share segments from both parents. Half siblings share segments from only one parent. Detailed segment data from tools that show both-sided sharing can separate the two more clearly than the total cM does.
Is a half sibling closer than a cousin?
Genetically, yes. A half sibling shares about 25% of DNA on average, while a first cousin shares about 12.5% on average. A half sibling is also a biological sibling in the sense of sharing a parent, though not a full sibling.
Next steps
If you find an unexpected half sibling, take time before contacting the person. Add a private note, check evidence, and consider that family stories may not match the DNA. Contact respectfully and let them respond at their own pace.
FAQ
Does Ancestry show half siblings? Yes, as close family matches, though usually without an explicit “half sibling” label.
Can a DNA test confirm the same father? A test can strongly suggest a shared parent, but identifying which parent needs shared matches or a tested parent.
Same mother, different father: are they full siblings? No, they are half siblings.