my guinea pig is biting my sister

Question
my guinea pig moody keeps biting my sister what do i do to keep her from biting?

Answer
You're not really giving me a lot of information but I'll do the best I can with what I've got.  

I had a young lady that I had given a pig to and she started having the same problem after she'd had the pig for three or four months. I tried handling the pig, poking at it, tried to aggravate it and nothing happened.  I asked her if she washed her hands before picking him up and she said no, she did not. Once she got in the habit of doing that the problem stopped.

It was the smell of food on her hands that the pig reacted to. He was not biting in anger, he was "tasting" her fingers and of course since he used his teeth to do so, she of course got bitten.

Have your sister try that to see if that helps. The other issue is to watch the way to pet the pig. You always stroke the pig in the direction that her hair grows. For short, smooth haired pigs like Americans the hair grows from the head to the rear.  In pigs like Teddies the hair grows from the rear to the head. Petting a Teddy from head to toe is irritating to them and they will sometimes nip.

I hope these suggestions give you something to work with and maybe solve the problem.