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My dad died 4 years ago. But my grandfather (his dad) could be my bio dad.

We have made a test with my aunt (his full sister) and we share 25,27% dna (1860,2cM 43 segments max136,2cM) With Geneanet we see that we share a common semi identical segment on our X CHROMOSOME of 73,3cM.

Is it possible to say whether she is my half-sister or aunt from looking at the DNA alone?

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  • dnapainter.com/tools/sharedcmv4/1860 suggests that autosomal DNA as a whole supports both aunt and half sibling so I think you should focus your question on whether your X-DNA can provide any additional information to help you distinguish the more likely relationship.
    – PolyGeo
    Oct 2 at 0:31
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In her post "Full or Half Siblings" on DNA Explained (posted 3 Apr 2019), in the section Unusual Circumstances, Roberta Estes says:

These cases tend to be quite messy and should be unraveled with the help of a professional.

and at the end she gives wise advice:

Regardless of the genetic outcome, I hope that you’ll remember that your siblings are your siblings, your parents are your parents (genetic or otherwise) and love is love – regardless of biology. Please don’t lose the compassionate, human aspect of genealogy in the fervor of the hunt.

While experts are welcome to post on Genealogy and Family History Stack Exchange, most of us are enthusiasts, and most of us are generalists. We are not going to be experts on every topic that is asked about on the site. Questions like this are best answered by professionals like the ones Estes recommends in the post I've linked to in this answer.

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