Sibling DNA tests are most often done as an alternate way to determine paternity when the father is either unavailable for testing or unwilling to test. There are different types of DNA testing that will accomplish this.
Because of the fact that the father contributes 50% of his DNA to a child and the mother the other 50% full siblings will share 50% DNA with each other and half siblings will share 25% of the same DNA.
A full sibling vs. half sibling DNA test will determine if the siblings share the same mother and father. If the test determines they don’t share both parents and if the siblings are certain they share at least one parent in common, which is usually the mother, then they can conclude the test results mean that they are half siblings.
However, there have been cases where the children have been adopted and they were never told they were adopted and they assumed their mother gave birth to them when in fact that was not the case. There have also been cases of in vitro fertilization where the wrong egg was implanted into the mother and the mother gave birth to the child and didn’t know that child was not her biological child. Other cases the children assume they share the same father which is more difficult to take at face value than a woman actually giving birth.
So if a full vs. half sibling DNA test comes back as excluded as full siblings it doesn’t always mean that they are automatically half siblings. To be certain you know the truth and have all the facts it would be wise to then order a half vs. unrelated DNA test. If that test comes back as confirmed then you can be certain you are half siblings.
This testing of course should be at least 24 – 25 markers tested for the most accurate results. Many companies out there offering DNA testing are only testing 16 markers and that often results in inaccurate results in the testing. It costs less for a company to test 16 markers rather than 24 or 25 markers and these companies are all about profit not making sure you have the truth.
These tests can be life changing to the persons ordering the tests so spending a little bit more on something so important and knowing the real truth is worth the few extra dollars spent on the higher marker testing.