Did you introduce them on neutral territory, or did you pop them in the cage together?
Regardless of this, then this is normal behaviour, its basically Pru showing the little one who's boss. I would recommend you give the cage a good clean so it smells of neither pig and then put them back in together. That way Pru won't have any territory to defend because everything will be neutral.
It doesn't sound like biting, a little nipping in normal, or she could just be sniffing the new piggie. When I first got Chewie (my little boar), Spike (my bike boar) would go right up to Chewies face and it would look like he was licking it, but he was just sniffing.
Introductions (well with boars anywa) often look very aggressive and when I was introduceing my two I instinctively wnated to seperate them, because Spike was showing lots of dominant behaviour and Chewie would be sitting there still just whining and crying. With my two I kept them seperate for about a week, and everyday they would spend a few hours together on neutral territory, by the end of the week the displays of dominance that took place on the neutral territory had practically disappeared and they were safe to be housed together. Apart from Spike chasing Chewie from pigloo to pigloo and the odd displays of rumble strutting at dinner time they were fine, and now they have completely settled down.
With sows it tends to be easier because in the wild they live in groups made up of a few sows and a dominant boar, once they establish the pecking order, and your new sow knows that Pru is boss they should settle down quite quickly. Just keep an eye on them for the first few days of them being together.
