I hope you and your cousins can talk with each other and figure out how much each inherited. Did you say that the current tax bill is just your part, but the earlier years have you all together?
It sounds like what should have happened is when your grandparents died, the inheritance should have been split then and the portions separated but since that didn't happen, you and your cousins need to get it fixed as soon as possible. It would be terrible if your or their interests were sold at a tax sale.