it shouldn't be expected that teeth will be used in play. When the puppy is with her litter mates it's expected and ok, but she needs to learn that we have now changed the rules and it isn't ok when she's playing with humans. You don't need to teach her to bite less hard, but rather not bite at all!
has she considered puppy classes?
We also have a BC puppy, who is now 5 months old, and was bitey when we first got her Puppies don't learn very well by being told off, rather they need to be rewarded for the behaviour you want. One technique you could use is to let one of the adults play with her (with a toy for example) Let her bite but ask her to stop when she does. When she stops, reward her with a treat, so she knows stopping biting is the good behaviour, or if she bites just stop playing with her and completely ignore her. Then she learns that if she bites she gets no more playtime. Neither of these worked too well with our pup so instead we used another technique which was to emit a high pitched yelp (as the litter mates would have done) when she bit. Then rewarded her when she stopped. This did work. my husband (and i think men in general owing to their vocal cords) didn't do the high pitched yelp very well though!