American College of Theriogenologists
My advice is to find a reproduction specialist; above is the website. And you have no time to lose.
You are in a difficult situation for sure. 7-10 days out from a whelp date... The fact that she is 5lbs is not as important as her overall health and the size of the puppies versus the pelvic ring. The vet should have given you an indication if the pups could safely be birthed.
The sister brother mating is very ill advised - I personally would abort the litter if the ovariohystercomy would not put her at undue risk. That risk being compared to going through with a whelp.... And with a brother sister mating you are doubling up on any recessive genetic traits that are with-in their line.
You definitely need an experienced second opinion from preferably a reproduction specialist.
On the vets attitude I am sorry but he was being blunt, whether he discussed the risks of a very late spay abortion versus whelping I have no idea.
Your gal is 3years old and you kept her intact - she has gone through at least 3-5 heats since you have had her. Surely as a responsible owner you have tracked those heats to know when she is next due to come into season. Yet you sent her off into a home with an intact male. Did she cycle early very early based on her previous heat cycles? Like it or not these facts might have colored your vets attitude.
As I said you are in a hard place with scant time to make a decision. For me that decision first would be on the medical risks to my female, second the health of any litter she might or may not successfully birth.