So my scenario is I have two characters who grew up presuming they were half-siblings (brother and sister), but for various reasons that relationship comes into doubt later. The male character was treated badly as a child by his dad and stepmother and so has no contact with them. How could he and his not!sister prove that they're not related through a DNA test that doesn't involve their parents? The background information is that the sister's mom was sleeping with another guy before she got married to the brother's dad and so he is the actual father and not the brother's dad, which makes him and his "sister" not really related. The mom has known it all this time, but has kept it hidden.
I searched DNA sibling test and DNA relationship testing, but I couldn't find anything conclusive as to whether it was possible to prove relationship between siblings without any DNA from alleged parents.
Also, what would the DNA test say upon a negative result? And what's the usual method, saliva, hair, blood?