I think Gma tries to minimize the effect of drugs on her family (drug busts, overdoses, etc.). Maybe she truly doesn't believe that her family's problems have any bearing on the disappearance of Lyric and Elizabeth. She may be trying to keep the focus on the missing girls and not the family, IMO. But she does come across as defensive sometimes, which I think is indicative of the pressure she is feeling right now.
I'm inclined to also think that Elizabeth was the target. It must be a tremendous burden knowing that this happened on her watch. It could have happened at any time, but it was while she was watching the children that they were kidnapped. I simply can't imagine how that must tear her apart.