My sister has an 8 month old mini dachshund that has accidentally gotten pregnant (by her father.) She is going to take her to the vet ASAP, but I’m trying to do research on the risks.
Can she die during birth or before? What can actually happen?

I have lectured my sister on this, so please no judgments, this was not me – I don’t even live in the same state... I’m just trying to get educated because I am worried.
Thanks in advance.

Her body is not developed properly yet, she's still only a puppy so yes she could die.

It's the same as a young human having a baby, there might be no problems at all or something could go horribly wrong.

Your sister can get a spay/abort done on the dog. It would be much better for her in the long run.
If she does decide to keep the litter then she needs to start saving some cash. The chances of needing a c-section done or higher with a younger dog and they can be very costly.

Shaz I have no knowledge on breeding myself, but what about the father mating with the daughter?
What are the risks associated with the inbreeding?
I know some say it is a useful way of fixing traits in a breed. I asked my mother and she said they very seldom used dogs from the same family or with that close of a "relationship" so to speak.

Thanks Aurora - Was actually wondering the same thing.

Also, I understand a C-section may need to be done and that it's risky, but what I can't find, is what exactly can happen? What type of complications?

Well from the research I have seen,
In small dogs who are still puppies themselves who get pregnant normally need c sections due to the fact the mom's are just that, still puppies, and the puppies in side of them are too big for them to deliver. Usually doesn't end good, and it much safer to have a sapy/aborty done.
If she does not want to go ahead with this then she will need to get her on a calcium supplement as well as a good quality puppy food until the puppies actually stop nursing. Much like humans, the babies "eat" up all of moms nutients.There is a fatal condition called eclampsia this is when the body does not produce enough calcium for her as well as the puppies. It can hit them extremely fast and kill them just as fast. One of the many reasons why you don't want puppies having puppies, as the body is using all the calcium it produces to maintain her bones, and doesn't have extra to make or keep up puppies. So to be on the safe side, get her on a calcium supplement as soon as possible as well as puppy food which also has added calcium in it. But I would try and push the spay/abort :(
GOOD luck!

Spot on Aurora.
In short the mother basically isn't big enough to handle puppies. She may look fully grown but everything inside her may not have completely developed yet. When the time comes for her to give birth the puppies just may not fit through.

And the fact that the mother is still growing herself she needs the nutrients that the puppies are going to be taking out of her. You will have to be very careful with what you feed, start her on a high quality puppy chow right now to get those extra nutrients.
Another risk associated with that is premature birth or underdevelopment. Because she is so young her body could decide to push the puppies out early. It really is heartbreaking sitting up all night with a dog helping it whelp only to deliver an entire litter of dead puppies.

As for the father-daughter mating I can't see there being any huge problems.
You do double up on hereditary problems which is a risk but I have seen several father daughter mating that produced healthy puppies, bear in mind though this was in show dogs (accidents happen) who have been health tested previously.

I agree with Aurora in trying to convince her to abort the litter, it's the safer option but again if she doesn't want to do that then please feel free to ask any question you or her might have about whelping and caring for puppies.

I personally have not bred a litter but my parents are show breeders and I have watched and cared for many newborn pups.

Thanks Shaz!
I hope the OP does come back for tips, and keeps us posted.