This doesn't make sense. Are you trying to say that putting bleach on a cheek swab for a paternity test will change the structure of DNA?? Why will your brother-in-law be reported soon? I doubt bleach will change the outcome of the DNA test anyway.
I believe there are two possible results of his "clever" trick: Either the test will work fine, even with the bleach, or they will be unable to get a result and will do it over. Bleach does destroy DNA, but it doesn't change it.