Act 19 - Since Calisto died in accident, as opposed to being murdered, there will be no one suspected of killing him, and he will have died in the most guilt-free way possible. Melibea's reaction to the aforementioned death seems quite normal, as most people in her situation would have grieved, although not to the point at which they would have liked to kill themselves.
Act 20 - Melibea's death is not a surprise after reading about Calisto's death and the dealings which occur post-Calisto between Melibea and her father. Despite trying to be persuaded to use a substance or other form of medication to help mend her emotionally and physically, Melibea's father's efforts are in vain, as Melibea continues to want to kill herself, regardless of the help she is offered, all due to the fact that she cannot bear to live without Calisto. Thus, she believes that if she is to die, she will be with Calisto in the afterlife. This seems like a fairytale idea to me, because the others who have died would also be in the "afterlife," resulting in the two never being peacefully together. At least, I believe their peers would be in the afterlife with them.