I wouldn't think of them as families in traditional sense either.
After I had left for college, my sister who was still in highschool, decided it would be "fun" to put Marshmallow, our female piggy, in with Mr. Spot our male piggy. Sure enough 3 guinea pig babies pop out.
Now here comes the kicker which kinda negates the "brother sister idea." If you do not remove the male baby guinea pig within 21 days of being born, he will impregnate his mother. How did we learn this
the hard way of course.
My sister doesn't take little Cherry out soon enough, and guess what, Marshmallow ends up pregnant by her barely 3 week old son. Needless to say the resulting litter of 2 was very, very white, just like their Mommy, and their daddy ummm brother.
In retrospect, we're really lucky no-body involved was a lethal white, (see the sticky earlier in this forum for more information) as Marshmallow was white and Mr. Spot was brown, black, and white. We're also lucky my mom teaches Junior high, and between her students and my sisters friends, we were able to find happy loving homes for all the babies.
We got lucky! Piggies do go at it constantly and can get pregnant during playtime when just visiting. They're like rabbits, in that their natrual predator defense is to breed. Its one of those defense mechanisms of that they can't catch all of us and if even two survive so will our population. A guinea pig herds natural response to a hawk is to stampede in all directions, to confuse it, but also so that it only chooses one instead of wiping out the entire herd. The herd can easily repopulate one missing guinea pig.
My personal opinion is Petfinder.com is a far better site for people actually looking to rehome their pets, and that if breeders are trying to sell their pets on craigslist they should be stopped. It's against the site policy, and when you consider, some of the people might just be buying them for snake food, a little gross. If they are selling them for snake food or profit, it should be stated up front and the guinea pigs should not be sold under the pretense of adoption.