If she wrote them, I hope it's to her three children. I hope she apologizes for departing the earth on her own, just briefly, though- she doesn't need to explain- 1/2 of the world will be willing to tell them, and not so nicely.
I hope that she told each child that they were special to her, that she loved them, discussed memories of them that made her happy and proud. Even silly stuff, like how they loved making mud pies. I hope she wishes them a happy, healthy, and bright future, that they deserve it. I hope she tells them that she loves them to the moon and back. For them, not for anybody else, but them. They deserve some kind of love and happy thoughts. They've already been through enough in their young lives, especially the last 3 and a half months.
I won't judge the ex if he doesn't, but I do hope one day, he lets the boys have a relationship with their little sister, providing she's not adopted out. The three of them have need some healing dow the road of their own. Their little sister won't remember anything, all she'll have is pictures to look at of her mother and plenty of news articles telling her why she no longer has her half brother Lucas, why she doesn't see her older half brothers as much or if at all. Thank goodness the boys have each other.