Peter and Sylar as twins is the easiest explanation, given that otherwise Nathan missed his mother having a pregnancy. They both could be the attempt to find a better form of Arthur's ability; the crucial difference to both Sylar and Peter is that when Arthur takes an ability, it seems to be removed from the original bearer. (The reason Sylar's victims don't survive is because unless they're Claire, they can't heal after he looks into their skulls, but as we see from Claire, him taking the ability per se isn't what kills them. Peter absorving abilities has no impact on the people he absorbs them from whatsoever, of course.)