My father and his 5 siblings inherited land when my grandmother passed away. The land was never divided among the six of them and he has been the only one paying the property taxes on all the land. Is there any way he can take sole ownership?

Offer to buy the others out, He needs to bill then for their share of the taxes. Of course does he use the property ? he may owe rent to them for use,

Is there any written agreement

The land was suppose to be divided but they never got together to do it, there are no written agreements. He has asked them to help pay the taxes and they have never given him anything. But he's been paying them alone since 1988 when my grandmother got sick. She died in 1989. It's a lot with the house he lives in on it. But the rest is just farmland. His brother and sister plant stuff on it. And with him getting older and having health issues. I worry if something happens to him, his will try to take everything from us.

Then he needs to have a will specifying what happens with his share. His siblings will owe his estate their share of the taxes he has paid, so he needs to have records showing he has paid them. Who's name(s) is in the deed. Was his mother's estate ever probated?