Is a marriage that took place under duress considered valid, and is it permissible for the sister to marry whomever she wishes thereafter?

It is not permissible for a Muslim woman to marry a non-Muslim, by the consensus of scholars. If such a marriage occurs, the marriage is invalid, and separation between them is obligatory without the need for divorce, based on the Almighty's saying: (O YOU WHO HAVE BELIEVED, WHEN THE BELIEVING WOMEN COME TO YOU AS EMIGRANTS, EXAMINE THEM. ALLAH IS MOST KNOWING OF THEIR FAITH. AND IF YOU ASCERTAIN THAT THEY ARE BELIEVERS, THEN DO NOT RETURN THEM TO THE DISBELIEVERS; THEY ARE NOT LAWFUL [WIVES] FOR THEM, NOR ARE THEY LAWFUL [HUSBANDS] FOR THEM) and His saying, the Almighty: (AND DO NOT MARRY [YOUR DAUGHTERS] TO POLYTHEISTS UNTIL THEY BELIEVE).

Therefore, the previous marriage is not considered valid. It is permissible for your sister to marry a Muslim man, and her guardian for the marriage should be her Muslim relative, or else the head of the Islamic center, or the mosque's imam, or any Muslim man. It is not permissible for her to conduct her own marriage contract. Efforts should be made to invalidate the previous marriage from the official records.