I guess is happends to the best of breeders ...if all was compatable and health testing done then i would let it be ..but agree with others if it wasn't a trip to the vet would be needed

someone i know has just had this ""brother and sister""!!!!! they were straight at the vets

If it happens to someone else = This really depends on many factors,the age of bitch and dog/compatability/health checks/owners experience/financial situation/work and home situation/the breed /if any of these are lacking/unsure I would advise the owner about the post mating injection.
If too late for avoiding action, be non judgemental, accidents happen and in these circumstances help and advise the owner for the sake of the bitch and puppies.
Myself it wouldn't happen, either a bitch would be spayed or dogs would be kept well apart. If it did happen there is no doubt about it, accidental means unplanned / not intended had it been a wanted event I would have mated the bitch so it would the vets and a post mating injection for me.

If I got pregnant by accident I would freak out and run for the morning after pill

If I had a bitch and she got pregnant by accident I would freak out and run to the vet!

I'm not ready for babies OR puppies!

But on a more serious note, I wouldn't let it happen. Archie is due for neutering soon, and if I ever got a bitch she would definitely be spayed as I wouldn't want to take any risks of pyo or anything scary like that. Also I would not want to put them both through the suffering of not being able to get it on when hormones take over as I have no good reasons to.