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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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There is a great difference between doing a relational DNA test as opposed to doing a genealogical DNA test as mentioned in the answer above

A relational DNA test actually compares one DNA sample against another whereas a genealogical DNA test looks at data from a database and gives you an estimation of what the relationship may be

Relational DNA tests however, are the only ones that can be used and are trusted in a court of law and therefore would be the best answer for a situation where you need to test one person against another

There is a company PaternityUSA that does offer a 46 marker test which analyses additional markers, and may help to increase the conclusiveness of the referred test result

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