How can one deal with a sister who ran away from her family and married, in a customary (urfi) marriage, a man of ill repute, and who has caused numerous problems for the family despite all attempts to reform her?

The marriage is valid as long as the father initiated the contract or approved it. Duress does not affect its validity, unless it was an unlawful coercion. In that case, the marriage is invalid. Therefore, one must pray for this sister for repentance and guidance, and it is not permissible to sever ties with her unless it is a permissible boycott intended for her betterment.