I'm sure everything will go fine! Since she is under a year old, there are less complications with the delivery unless the babies are very big! Is she a very tiny guinea pig?

I was thinking about having her sister in with her, too. Maybe you could some how divide the cage? What type of cage do you have? With a C&C cage you could divide it down the center so they could still see and feel each other, but her sister wouldn't come in contact with the babies.