You don't understand how DNA tests work. There is never a 0% or 100% result. It's measured in "probability".
Or another man, you don't say if the girl is having sex with more partners.
Hold on here. Are you referring to a PATERNITY-test to figure out who is the biological father of the girl's child?
In principle it is possible that the child shares zero chromosomes with its biological uncle (brothers typically share about 23 of their 46 chromosomes with eachother), but the chances of that happening are extremely small
No. Brothers are 50% related, not 0%.
Only if the other brother was not a blood relative.
If the brothers had the same mother or same father (or both) there would be at least a small relation to the child.
So either they are not blood relatives or she slept with another guy.
if it is 0% ( not actually possible) it means the father is a third party
Human beings are 99.6 percent similar. Therefore, it can never be zero percent.
That's not a realistic scenario because DNA paternity tests never show "0 percent relation." Never. That's not how they work. First of all, DNA tests are never absolute. No DNA test ever gives 100% or 0% as a result. Second of all, DNA paternity tests say something like, "There is a 97.38% likelihood that the subject whose DNA was tested is not the biological father of the other subject whose DNA was tested." That likelihood does not mean that the subject is not related to the child as an uncle. You cannot draw that inference. It is not giving a percentage match of DNA but a percentage probability that the DNA tested is that of the father of the other subject whose DNA was tested, not the percentage probability that the DNA tested is that of some other blood relative. If you want to know if the other man, the first man's brother, is the father, that will require a separate DNA paternity test, for it cannot be concluded that he is not the father from his brother's DNA paternity test. That said, whoever performed the DNA paternity test would certainly know that while there is a 97.38% likelihood the subject is not the other subject's father, there is a likelihood that he's a blood relative of the child, but laboratories performing DNA tests are strictly controlled and are not permitted to editorialize in such a manner in official results, the results only stipulating to the relationship that was tested for, meaning you would never know that the tester knew that they were blood relatives, not unless you included that in the scope of what was being tested for.