I would prefer to try to convince her not to lie to the family and most likely would not lie for her. At some point the truth comes out or someone will find out what has happened and its just messy all around.
I wouldn't change her mind about the adoption if that's what she wants to do- some people are well aware that they wouldn't be able to take care of a profoundly disabled child and I wouldn't begrudge them that- but I don't want to lie for them and especially not with something so serious as the death of a child.
Haart: um is that the selective reduction? Does that only work with fraternal twins or can it be performed with identical twins?