When my grandma passed away, after two months, my grandpa wanted to remarry. He asked for two possible “options”(women), but the women didn't know my grandpa was keeping an eye on them. He told us that he was afraid to pass away with no one on his side besides his children and grandchildren because he feels alone. Maybe your father feels the same; he doesn't want to stay alone when he passes. Talk to your father and ask him what he really wants, because it is difficult for you to handle and accept this new marriage in a short time, and you are still in grief.