Hamster inbreeding: does it cause birth defects?

i have a surprise pregnancy on my hands, as we were told the two hamsters we got from the pet store were male, and i'm now pretty sure ones female ones male (plus, i saw them "mating"). i'm extremely worried because the pet store told us that they were from the same litter, and i don't know if breeding brother and sister causes birth defects. can someone please inform me, so i know wat to be prepared for come time the babies are born?