The chances of mom getting pregnant is very close to 100%.
The boys are generally mature and capable of breeding by 2 months of age although some may be fertile earlier. Females mature at 6-8 weeks although in your situation, the female is already mature. Most people with experience recommend waiting until 5-6 months of age before neutering. This means that you will need to separate the boys and mom shortly.Surgical neutering is the safest form of birth control although separation of the sexes is effective so long as no one accidentally puts them back together.
Surgical neutering of males and females is similar to the procedure in other species. It requires that the surgeon knows the variations in anatomy and physiology of the species. Because testicles are external and ovaries and the uterus are in the abdominal cavity, neutering females is a more involved procedure. That again is the same as in other species. Getting the female neutered (spayed) may be the best option if you want to attempt to keep the gang together.
While GPs are social animals, your boys may not get along particularly as they become mature. You may find that you need to break up the group at some point.
GPs are fun animals and I understand your attachment to them.You need to find a good vet for our GPs. You are in the Philadelphia area and there are many vets with expertise in exotic pets. Your vet should be able to refer you. You can also look in the Yellow Pages or on the internet and then see if your vet would recommend someone from the list. The Penn Vet school may also have a list of vets who handle exotics.
There is a lot of information about piggies on the internet and a Google search will find info on all aspects of husbandry.
Let me know if you have follow up questions.