I'm not sure how all the little bits an pieces come together here. To me, the key fact is:
If true, grandfather's estate needs to be probated before anyone has any rights or responsibilities. From a later post, it seems the term "intestate" was used. If the property was separate property, I believe 1/3 would go to his wife and 2/3 to his children (or descendants of children) with wife having a life estate in the home.
But, nothing really passes unless probate or, if less than $50K in value, an affidavit transfer occurs. If there was a transfer, grandma has gifted her rights to the OP and brother and it is theirs now. If there was not a transfer, the first probate/affidavit must happen and then, probably, the transfer takes place. I say probably as the type of deed and how it was written may be important. While grandmother might be estopped from challenging OP and brothers ownership, it might not have been validly transferred.
As to the taxes, the owners are responsible.