I won't agree you legally own it at the moment since it appears his estate has not been examined to determined whether it would require the sale of the motorcycle to pay estate debts or that grandma may have some unknown claim to it . . . .
I am also disinclined to agree. Dad's estate may or not be solvent, but seldom does a person die without having some outstanding current, financial obligations.
In that respect it would be interesting to know on what basis the siblings in tendering Form VTR-262 (Affidavit of Heirship for Motor Vehicle) verified under penalty of perjury that there was no necessity for an administration of their father's estate!
Because, short of someone forging dad's signature to the Certificate of Title, Form VTR-262 is the only means whereby title could be transferred.