ribit,
You're on target, drug addicts & alcoholics are capable of doing many things under the influence.
If they are addicted, the other children should be watched, maybe placed in foster care, until or unless they get help.
But if she was sold, folks near the new parent would notice, don't you think?
Unless the child was taken out of the country?
Stay on the case, I'll try to do keep an ear to the ground.

DO SOMETHING!