Can children inherit from grandparents what was left to their dead mother?

I am so sorry for your loss.  I think that you should seek help from an attorney not associated with the estate.  You are in Illinois, correct?  The intestacy laws in Illinois state that the beneficiaries would be the children of your grandparents "per stirpes" which I was taught to mean "through the roots" meaning your Mother's share passes to you and your siblings.  The taking is by representation or by class, so your Mother was a class of beneficiary - descendent of the decedent - so if one of the class of beneficiaries who is closest in relationship to the person making the distribution is deceased and survived by any descendants - you and your siblings - then that deceased beneficiary’s descendants will take “by representation” what their deceased parent would have taken.  Good luck to you.