Is helping a sister repay her usurious loan, to which she was compelled after leaving work, considered participation in usury?

Borrowing with interest (riba) is one of the gravest forbidden acts, and it is not permissible to assist in it except in cases of necessity. However, what you did does not constitute assisting your sister in borrowing with interest. If your sister does not repay the principal amount of the loan, then your assistance to her in repaying it is permissible and a righteous deed, because repaying the principal amount of an interest-based loan is obligatory. What is forbidden is voluntarily repaying the usurious increase.