You know what you feel you ought to do with the ring, so why not just do it?
The only reason for not giving it to one of your cousins, as far as I can see, could be that you are afraid the other two might be hurt.
If this is what is holding you back from giving the ring to one of them, I suggest you give it, saying that it is the only thing you have that was your and their grandmother's and that as you have no-one to leave it to, and you would like it to remain in the family that you hope one of your cousins, or their daughters would like to have it.
I had a silver bangle given to me by my paternal grandmother. I sent it as a present to a little great.niece on that side of the family, and it was greatly appreciated.