Spoiler
Based on the epilogue of the novel, its a theory that the current timeline is Sienna's "dream". Cause in the dream timeline where she & kun are not in good terms, she actually dies early (~45 years old? I think). Right before her death she met "God" & he asked if she had any regrets/wished for anything. She said something along the lines of "i want to make my loved ones happy"/"i want to save my loved ones" (can't remember 100%). So "God" grants her wish, hence why the title is The Great Wish. Its unclear if her current life is a dream or not, but ppl assume that cause of the epilogue.

Y’all really making me not wanna read this anymore huh???
I heard y’all say sum that this is all in her dream? And if that’s true, get outta here with that sad shitI’m runnin away