1. If you are both males, then a Y-chromosome DNA test can prove if you share a common male lineage. The Y-Chrome test does not prove if you have the same father, because all males from the same family share the exact same Y profile. Therefore, you share the same Y-Chrome profile as your father, your brother, your uncle, your grandfather, your son, and so on. With a Y-Chrome test you need to be certain that other family members are not potential fathers.
2. If your half sibling is a female and you are a male, or vice versa then a half vs. unrelated siblingship test may be right for you. Depending on your genetic makeup, the half sibling test can be extremely conclusive. There is also a chance that the results are fairly inconclusive. A situation where results would be very conclusive is when you and your half sibling share very rare genetic markers. Since this is unlikely to happen, the probability of you being related is much higher.
One way to improve the conclusiveness of a half sibling DNA test is by including one or both mothers. This allows the DNA technicians to exclude the half of your DNA that is contributed from your mom, and isolate the half that was contributed by your father. Once it is isolated, it is easier to get a more conclusive result.
Our laboratory guarantees DNA test results that are greater than 99.9% accurate for all paternity tests. Although we do not make the same guarantee for half sibling tests, there are many things we can do to improve the conclusiveness of your results.