Nanwind, You say your DiL had difficult childhood. As a result of this childhood, she probably has deep feelings of worthlessness, that no matter how nice you are to her she cannot believe she really deserves it and can only explain your kindness to herself by seeing you as doing it for a purpose - and that is; you are nice to her so that you are able to see your DS and DGD.
Most of the time she keeps those feelings to herself, but alcohol can remove inhibitions and all of a sudden all her feelings of worthlessness spilled out. This belief that you quizzed your DGD; you probably asked her a very innocent question that she mentioned to her mother who immediately misinterpreted as an effort to criticise her.
I write all this, because I know of what I speak. I had a kind, gentle and loving uncle, but he too had had a difficult childhood without love or affection and his feelings of worthlessness were so deep that despite a successful career and happy marriage to a wife who understood his demons, he could never believe that any one could admire anything in him and saw every compliment as thinly veiled criticism. His life was one of constant fear of being revealed as the worthless person he thought he was. Nothing that all the people who knew him and loved him could do or say, could ever change this sense of utter worthlessness.
Your Dil sounds like my uncle. So be kind to your DiL, ignore everything she said. She spoke from her internal misery, rather than with any unkindness to you. Have a quiet word with your DS and tell him you understand the internal causes that led to the outburst and you will forget about it. Have pity one very damaged soul.