A pair of litter-brothers or of litter-sisters will get along well together without adding to the guinea-pig population, too, without being neutered. However, once the brothers meet a female and find out what they have been missing, they tend to dislike one another. Two sisters will agree for life.
It has been known, though, for what were thought to be the same sex to turn out to be a breeding pair. The babies are just gorgeous, born with fur and open eyes and running about at once. They are milk-fed for a few weeks, but they start to nibble adult food in a day or two, and should be gently handled from the start to get them accustomed to humans.