How is it that you can verify someone's DNA,as opposed to say, their brother, without their own DNA? So what if they say they had his sister's DNA to compare to

How is it that you can verify someone's DNA,as opposed to say, their brother, without their own DNA? So what if they say they had his sister's DNA to compare to? It could be his brother who was living there and died. How does that prove it was him? I'm honestly asking this, not trying to be difficult. I know photos can be photoshopped, but everyone can debunk those. Obama should show his death photo and let us decide.