Is it forbidden for a doctor to take free medicine from the hospital pharmacy for his well-off relatives, other than his allowed family members? And what must he do to atone for that?

If what is authorized is taking medicine only for the wife and children, then it is not permissible to take it for the mother or siblings. Whatever the questioner took, he is obligated to guarantee its return, either in kind or in value, to the responsible authority. If he is ignorant of its quantity, he should strive to clear his conscience. He is not required to inform them of this. He must repent, seek forgiveness, and resolve not to return to such an act, for it is a betrayal of trust.