K.K.
I am not sure, but I don't think you should be testing your brother's kids without the parents' consent. This is really not your place to do. Just my opinion, I am sure there is much more to your story but still, if he is not willing to have it done it is not your place to do so. Also, if he is married to the mother, he is still LEGALLY the father regardless of what the DNA says. So, what is the point of interfering? I recommend you talk to him and don't do anything behind his back that he is not aware of.
Good luck to the children involved, whatever you decide.