Where did you get these guinea pigs from? What do you know about their background, genetically? Genetic diseases, etc? How many generations does their pedigree go back? And how many shows have they competed in and won?
If you can't answer these questions, you are not ready to breed your pets. How would you feel if, due to genetic diseases, the babies were all born ill, or dead? What if your sow died in labour? I know that it is harsh, but if you are breeding BYB or petstore piggies, it is highly likely.
Also, do your cousins already own pigs? If not, how much research have they done? How much money do they have saved up for vet visits... which are almost inevitable if the pair that you are breeding are from a petstore, BYB, or anywhere other than a breeder that gave you SHOW quality pigs, not pet quality pigs. If they don't own pigs, they can't have one each. As I'm sure you know, guinea pigs, with very rare exceptions, NEED companionship in order to thrive. Personally, I feel that it is cruel to keep them alone.
Please, PLEASE leave breeding to the people who know what they are doing, who have studied genetics and have good show line pigs and only breed healthy guinea pigs. Just because your pets look healthy doesn't mean that they don't have an underlying health problem that may be passed on to the babies, and I am sure that you don't want that to happen. I am also sure that if your cousins want guinea pigs enough, they will be able to find a rescue that has guinea pigs. Some rescue centres would be happy to arrange transport if they feel that the piggies will be going to good forever homes.
Also, please don't be offended by this post, or any others. I am very passionate about rescuing, and write this only to educate... why breed and buy while shelter animals die?