Inbred baby guinea pigs--hair is different from that of parents and one sibling--why?

The Rex and the Teddy both have short, kinked coats, and that's likely what you are describing. They are recessive traits, which could explain why it's shown up in a litter from two related guinea pigs. If you haven't already, make sure and remove the male as the female will be receptive to breeding again often within an hour of birth and you'll end up with another litter.