My grandmother died on 3/6/20 according to probate her eldest son children and my mother legally make decisions. I have assumed the mortgage payments

By "according to probate" do you mean "according to her Will"? Regardless, the only instance in which you need not open an estate in the local probate court is when someone has only left personal possessions of de minimis value (no action) or those and a house without a mortgage (Affidavit of Heirship -- which the courts will not enforce for five years to let any "accidentally" omitted heir object).  You cannot simply take over the mortgage payments and claim the house for yourself or anyone else.  The lender extended the loan based on your grandmother's credit, not yours.