The same thing happened to my parents. My brother had a daughter and both him and his wife became addicted to drugs. Eventually my parents had legal guaradianship of her and raised her for 8 yrs until my sister in law got her act together - took a long time, my brother never did get his together.
Your grandson is adorable and he is very lucky to have you to take care of him. We will be keeping your daughter in our prayers that she gets the help she needs!!