autopsies have already been performed....this is a grandmother who in her grief is acting in vengeance against her one time daughter in law
Setting aside the fact that the mother is a horrible human being who would deserve any "vengeance" she had coming to her, this may not be about vengeance.
If the grandparents are Catholic, they likely view cremation as highly disrespectful.
It seems much more likely that her desire to burn her children is more about spite against her in-laws, than her in-laws desire to give their grandchildren a traditional Christian burial is about spite against her.
Sounds like the toddlers likely died from carbon monoxide poisoning. Did the autopsies show a cause?