This all starts out how it does in the first Twilight book. Bella's mother remarries Phil and she wants to travel with him. So, Bella goes to spend time with her dad in Forks. The only difference is that the Cullen's have moved away before she gets there. Bella meets Jake when her father buys her the car and they instantly become friends. The twist to this is that he is already a shape shifter and she is his imprint instead of Renesemee from the fourth book.
Eventually, Jake tells her everything about what he is, with permission from Sam because he is the pack leader. In the book he is meant to be the pack leader but he doesn't want it, so that is essentially what happened in here too. And like how Bella accepted what Edward was, she accepts Jake the same way. They start dating a couple of weeks after she finds out. Three years into the relationship Jake proposes to Bella. (Remember Jake was a shape shifter when they met, so he hasn't aged.)
Jake asked her to marry him literally a week after he turned eighteen - he was still fifteen physically - and Bella was twenty. So, even though they actually have a two-year age gap between them it really is more like five-year age gap. She has reservations about marrying him, mainly because of how her parents ended up, but she eventually says yes. They get married a month later in order to give Renee enough time to make it to the wedding. They have the ceremony at the courthouse with a small party afterwards because Bella did not want a huge wedding. Jake then stopped phasing after they got married so he could grow old with Bella.
Two years into the marriage they have their very first child, Melanie Claire Black. Four years later they have her twin brothers, Eric David Black and Cole Michael Black. When Melanie turns eight and the twins are four, Bella and Jacob decide that they should move because Melanie is being bullied on the reservation. They move to Kentucky because of the distance. Their life is a lot happier once they move and it stays that way for the next nine years. Bella is now thirty-nine and Jake is thirty-seven, at least from a legal stand point.
Technically he is thirty-four because he wasn't aging for three years. Melanie is seventeen and the twins are thirteen. Bella soon finds out that she is pregnant with their fourth child and wants to move back to Forks so it can grow up knowing its family there like her other children did. But shortly before they leave for Forks, the Cullen's move back. (So, just to help it sink in, this story is set in the future. Every human teenager from the actual books are in their late thirties.)