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My sister and I suspect we have different fathers.

We took a siblingship test through DDC. The results are a little confusing and honestly, they put our wet swabs each in a paper envelope and then packaged them together to send them to the lab. The siblingship index says 357, and says 99.7% (WITHOUT mother’s dna). The lady I spoke with from DDC said she has never seen it at 99.7% before and that is extremely high for a sibling test.

Should I be questioning this or am I being ridiculous?

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The siblingship test looks for DNA markers and can determine close relationships. If the company told you your results are evidence of you and your sister being full siblings, then it's likely that is the case.

To look at your DNA more carefully, and in context with other matches, your best choice is to use an autosomal DNA test from one of the major genetic genealogy providers (at this writing: FTDNA, AncestryDNA, 23andMe, MyHeritage). Then upload your raw data to Gedmatch for a fantastic segment mapping tool.

If you're not sure of the results of the DDC test, then try a different test (they frequently go on sale...keep your eyes open). You can tell the difference between full and half siblings from the cM total. Should you be in the small area of overlap, you can also tell by examining the segments with a tool (like Gedmatch has) that allows you to see full and half identical segments. Full siblings will have multiple areas that are fully identical; half siblings will have none (or maybe a blip).

If you're concerned that you and your sister may have a genetic relationship that is not standard, an autosomal test may help you suss that out as well.

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