Chances of getting the stomach virus?

Last night, my sister let me eat the rest of her pasta. I didn't use her fork. But a little bit later, she started getting severe stomach pains and after hours of crying in pain, she finally vomitted... A lot.

In about five and a half days, I'm going to have my friend stay with us for two weeks, and I can't really afford to get sick.

This morning, I started to drink grape juice in hopes that it helps a little, or at all.

But what are my chances of getting her sickness? I have not been exposed to her in any other ways beside that, but I'm very worried. I also wash my hands very very frequently (and always have).

Please let me know, so I can take precautions, or be prepared. Thanks :)