She's not. When I say "estate" I don't mean some sprawling Biltmore-type thing, just "estate" in the meaning of "everything she owned"; her house, her possessions, etc. Not a vast fortune, but certainly worth bribing someone to help you inherit.
Summary: Biff is still too young to place bets himself, so when his grandma goes to Vegas to gamble, he goes with her, gives her some money, and tells her what teams to bet on with his information from Gray's Sports Almanac. But she thinks she knows better and bets on what teams she thinks are going to win, thus wasting the money Biff had given her. The second time, she places the correct bets, but decides she'll keep the money except for what Biff had given her in the first place
They get into an argument that a mobster named Lou happens to overhear; then she goes into a casino that won't let Biff inside for being too young. Lou realizes Biff had correctly predicted the games, including the ones where the underdogs had won. He realizes Biff has some kind of inside knowledge, so he talks to Biff, saying that he has a knack for sweet-talking old ladies, and he'll persuade her to give back Biff's winnings, in exchange for the two of them working together; he'll place the bets that Biff can't because of his age, and they'll split the money in a more equal fashion than what Biff was getting from his grandma. He agrees. Lou goes into the casino to talk with Biff's grandma, saying she won a secret payout from the slot machines, that they can't give to her on the floor, but if she'll come back with him to the back room, she'll get $10,000! She agrees...
Then we see Lou walk out of the casino, giving Biff his money and saying he left her playing slots. They place their first bet together; Biff correctly predicts a knockout in a boxing match, and Lou sees him consulting the sports almanac. They collect their winnings and go to Lou's car, where the mobster hits him in the head and knocks him down. Lou opens his trunk, where we see Biff's grandma's dead body. He pulls out a tire iron, saying he'll kill Biff too, take the sports almanac, and get rich all by himself. He says he's not completely without mercy; he'll bury them both in the desert together. But Biff isn't as injured as Lou thought, they get into a struggle, and Biff ends up beating him to death with the tire iron.
Interestingly, we see Biff actually feel a little guilty here, being horrified that he's become a murderer. But after a moment, he starts rationalizing it to himself that it was self-defense. He buries them both in the Vegas desert, then goes to a document guy that he knows, bribing him to forge the death and cremation certificates for his grandma so he can inherit her things.