Can I break the sac on a pup that has been delivered and mom won't break it

Yes!

I hope you have already done so or the puppy will have drowned by now.
Sacs needs to be broken straight away, some dams don't have a lot of maternal instinct and personally I don't like letting them do ALL of the work in case something goes wrong.
Cords should be cut by you not the mother as well, you can do this with some sterile swabs, some cotton and some sharp scissors.