It's twofold, I think. One that they are grieving and two that they are still angry with their mother's actions.
Frankly, if my husband left me at a time I needed him for another woman I'd be very angry and at their ages, I would have a hard time not expressing that disdain now and again. We try very hard not to badmouth my stepkids' mother but I know she doesn't do the same in return. The facts as you present them is that she DID abandon them and why wouldn't they have their own anger about it? If therapy was not offered then, then that was a failing of both parents.
I think they all need family counseling, too. She's returning to their home that they shared with their father after a huge trauma compounded by their father's loss. She is also stepping back into a role she abandoned. They will need more than words to trust her again. She has to earn their trust and respect back. She cannot expect to wave a magic wand. Anybody can be a mother or father, but it takes a lot of work to be a parent. These children have been hurt and they need direction for their hurt. If their hurt includes being abandoned by her once, then it needs to be addressed, even if is hard to hear. My father was not a good father and abdicated his responsibility toward his children. The fact that my mother didn't like him was one thing but truthfully HIS actions (or lack thereof) were why I didn't cry when I found out he'd died in 07. The mother has to own her own behavior, too, and not just blame their father's reaction to her actions. Their father's behavior may not have helped, but that doesn't absolve her. If the only reason she's there is because he's dead, I'd be pretty angry, too, in their shoes. Where was she prior?
They may not want to talk to the hospice people but they should be encouraged to speak to their school guidance office, or to attend group sessions with kids their own age.
All of these things are not going to be repaired overnight. Everyone needs to acknowledge that, and not just blame the dead.