Back in X-Treme it was posited that her DNA does indeed catalogue all the powers she absorbs, and under certain circumstances, she was able to recall those stored powers at will. I would think, given the genetic nature of mutant powers to begin with, that Rogue is able to reverse-transcribe the DNA of her target to her own(at least epigenetically). But that is a good point, would Sinister's DNA sample be as constantly updated as necessary to keep her newest absorbed powers(I would think not, but then its also possible for some sort of "spooky-action-at-a-distance/quantum entanglement" effect to be at play and it could update on its own)?