Some types of close relatives, like aunt, uncle, half-sibling, nephew, niece, grandparent, and grandchild have very similar patterns of shared DNA, making predicting those relationships from DNA alone difficult. That’s why some customers might see someone incorrectly labeled as one of these relationships. To help improve detection of these relationships, 23andMe is now using a new technique that analyzes DNA from full siblings
Telling these relative types apart is especially difficult because the amount of DNA each type shares is almost identical. Because of this similarity, Bonsai relies on customers’ self-reported ages to make a decision about their relationships.
You can use this updated Bonsai method by rebuilding your family tree. To do this, you can request a recalculation of your tree through a link in the DNA Relatives FAQ page, in the question called “How do I recalculate my tree?”
The 23andMe database currently includes over 2 million customers who could be affected by this update, which translates to many thousands of relationships potentially changed. For those of you with a full sibling and a predicted half-sibling, aunt, or uncle, the updated Bonsai may produce a different (and more trustworthy) tree.
Keep in mind that unfortunately your tree annotations don’t carry over when you rerun Bonsai. Still, some of you will see changes and the predictions Bonsai now makes are much more reliable for the close relationship types discussed here.
the above is from 23andMe on Feb 22, 2023