Snow does look female to me, but @
ThePigAlchemist gives good advice.
There is a very definite possibility at least one of them is pregnant if they were kept with males for any length of time.
Have you gotten a scale to weigh them? A small kitchen scale (available for pretty cheap at places like Bed Bath & Beyond, Target, Amazon, etc) is invaluable for piggie care, as changes in weight are very often an important indication of illness or injury. It is recommended that you weigh them weekly (more often if you suspect sickness). Obviously, a pregnant sow will gain weight.
Around six weeks (correct me if I'm wrong on timing, folks), you should be able to feel the babies moving if you press *gently* against her sides. Closer to delivery, you'll also be able to hear the babies gnashing their teeth if you listen.
As for their origins, I say don't beat yourself up over where you got them. I 100% agree with @
bpatters about adopting any future piggies you may acquire from a local rescue or shelter, but what's done is done and now you know. There are more than a few of us on here who bought our first piggies from pet stores without knowing any better. I have learned a great deal from the people here and the importance of adoption was one of my first lessons; when I got another pig, it was from the shelter.
I am going to keep my fingers crossed that none of your girls are pregnant (but hope for a safe and healthy pregnancy and delivery if they are).