Should I invest in getting a DNA for myself, or for my 4 grandparents? Even then, there's an issue...

Hello everyone, freshman to the genealogy world here.

We are from, and live in the Dominican Republic, like most families we have a history that traces back, by genetics and name alike, to Africa and Europe both.

There are little records kept before the 19th century, but that is a problem I hoped to overcome by narrowing down our origins with a DNA test.

It ocurred to me that doing it for the oldest living siblings is a faster way to narrow down family origins than taking the test myself.

Problem is-- out of my 4 biological grandparents, only one grandma is alive. With the little genetic knowledge I have, I assumed that a full-blood sibling (son/daughter of the same father and mother as my biological grandpa/grandma) of the others will suffice instead.

New problem then-- only my other grandma has a full-blood sibling alive (3 really). My grandpapas do not, only half-brothers and half-sisters are alive.

Is it worth it then to run the test for the half-brother or half-sisters of my grandpapas? Is it worth it to go through this trouble at all, would that point me in the right direction to build our family tree?

Thanks for your input!