Let's say both cousins are related through their grandmother on their fathers' side, but neither have parents or grandparents still alive... so that rules out the possibility of getting a dna sample from them. But if 2 cousins were to take a dna test themselves, is it possible that they could determine whether or not they had the same grandfather on their fathers' side?

Since I was a chlid my grandmother on my moms side said that we have crow indian and blackfoot indian heritage. I have found nothing to support her claims. How and where could i take a dna test to find proof of or disprove this family claim?

We have tried 3 times to set up a "civil" DNA test with the mother of my brothers (we think it is his) baby. And everytime we have done this the grandmother of the baby puts a stop to it (the daughter is 17). My brother is wanting to step up and do what he needs to if the baby is his, but like I said they keep stopping our atemps.

Let's say both cousins are related through their grandmother on their fathers' side, but neither have parents or grandparents still alive... so that rules out the possibility of getting a dna sample from them. But if 2 cousins were to take a dna test themselves, is it possible that they could determine whether or not they had the same grandfather on their fathers' side?
ok, I realize that, but if they had the same grandparents, wouldn't 50% of the cousin's blood be similar enough to warrant an answer?