I'm getting another guinea pig tomorrow..?

I got my current guinea pig last Friday. It's a female Agouti and white(: Her name is Pappy :D I love her dearly. She's the best pet I've had in a while...Within the past few days she's become very inactive.. :( I think she's lonely. Her "cage" is a storage bin with holes in the top..I know that's wrong but it's the best I could do for the time being. I found out that you needed to keep guinea pigs in pairs the other day, because the lady at the pet store failed to tell me when I got Pappy. :/ I talked my parents into taking me back to the pet store so I could get another piggy:D It's been a week, so what are the chances they'll be from the same litter? Are they going to recongnize each other? Is there anything I should be cautious about? Also I'm getting a wire cage tomorrow too..what size will I need for 2 guinea pigs? Thanks for your help(: