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Once upon a time, when Jack was human, there was this dark entity targeting his sister for whatever reason. Maybe it wanted her for a sacrifice, maybe it wanted to eat her, the reason doesn't really matter and is ultimately up to you.
Jack saved Pippa, of course, because he's a good brother. But he didn't do it without seriously pissing off said dark entity, because he's Jack.
So it cursed them, and this petty dark spirit was particularly wrathful and creative when it did so. The siblings were doomed to die saving each other over and over again.
For example, in this life, there would be a bear attack, and Jack would save Pippa by tackling her out of the way and taking the killing blow, leaving her to live the rest of her life in sorrow and guilt. Then they get reincarnated as siblings again, with Jack still being older than Pippa. They grow up until Pippa was around the same age she was when she got cursed, and then they're in a fatal accident together, although they haven't died yet. They suddenly remember their last life and the curse, and this time it's Pippa who saves Jack by taking his place and dying for him.
And then they reincarnate again, and the same situation repeats.
Again.
And again.
And again.
So for infinity, they're stuck living their lives until they get into a circumstance where either of them could die and they take turns saving the other, because they remember how they die and trauma stacks, even if they don't realize why they're specifically scared of bears or fire or knives until they get into another life or death event.
Then in their latest lives, they go skating and just as the ice is breaking they remember all the times they've been in the same situation. By this point, they're not really scared, they're more resigned if anything, although they still argue over who dies this time because they'd rather shoulder the trauma instead of their sibling since arguably living the rest of their life in peace even without the other one there is made more bearable by the fact that they know that they'll reincarnate to be together again and restart this whole cycle.
Anyway, Jack wins the argument and dies, and Pippa resigns herself to living out the rest of her life without him again because he tends to be the one who dies the most often.
But then she reincarnates into her next life and to her shock, she not only remembers everything from the very beginning, but she's an only child.
Apparently the man in the moon temporarily broke the curse by turning Jack into a winter spirit without his memories. So for the next three hundred years, Pippa is left to live out different lives in a panic, wondering what happened to Jack and if she'll ever see him again.
Cue the movie where she's reincarnated as that girl with the beanie who's friends with Jamie. And she meets Jack, who has his memories back. They do a whole reunion thing which confuses everyone except the other Burgess kids because Pippa told them about her whole reincarnation thing to explain why she acted older than her age although some of them didn't believe her until this moment.
But there's still Pitch to fight, and just as he's being dragged into his lair by the nightmares, he panics and shoots nightmare sand stuff everywhere, and a lethal amount starts sailing in Jack's direction just like how Sandy died in the movie.
Remember, the curse was only temporarily broken.
Pippa jumps in the way and takes the shot instead, which means she's dying now.
Everyone panics and Jack yells at the man in the moon to do something since he's a spirit now and he's terrified of what the curse could do to his sister because of it. Magic doesn't always tend to mix well, after all.
Thankfully, the man in the moon does something.
When Pippa dies, the man in the moon turns her into an immortal spirit like Jack, although she doesn't have ice powers and instead controls nightmare sand and shadows like Pitch does. And at first she doesn't remember anyone, but Tooth fixes that quickly.
So now Jack and Pippa get their happily ever after with the curse broken, and Pippa becomes a guardian of fear/safety to take Pitch's job as the boogeyman since he was obviously abusing his position but everyone still needs a healthy amount of fear to survive.
Coincidentally, Pippa means 'lover of horses' and she's technically the nightmares' boss now so that's pretty neat.