Baby boar getting mom and sinblings pregnant?

My two female guinea pigs had babies, and there was one boar in the litter, the runt. He started mounting his sisters and mother, and I sepperated him at four weeks old. Is there a big chance he could have got his mom or sisters pregnant? What is the very earliest age he could have impregnated them? Because he was the runt, does that make the odds any smaller?