Is there a sin upon her parents for preventing her from fasting as a child based on doctors' reports? And must she make up all the missed days of fasting since puberty?

Illness is one of the valid excuses for not fasting, and the ruling on fasting for the sick person oscillates between being disliked (makruh) and forbidden (haram). A sick person should not break their fast except with the testimony of a trustworthy and specialized physician. If the illness is chronic, the person breaks their fast and feeds a poor person for each day. As for a temporary illness, the missed fasts are made up after recovery. If physicians advise against fasting and the illness is not expected to be cured, then feeding the poor is obligatory, and there is no make-up fast. However, if physicians now affirm her ability to fast without harm, then she must fast Ramadan, and she has no excuse for not fasting.