In my opinion, since this debt is a debt during their marriage relationship, although your sister did not use the money, it can be regarded as a personal debt of husband and wife, but this debt cannot have a confrontational effect, which means that your sister is still obliged to repay this debt, but after repayment, your sister can recover it from your former brother-in-law.
Unless your sister and brother-in-law have an agreement on the duration of the marriage relationship and can prove that a third person knows this agreement, your sister doesn't have to repay it.
According to the provisions of the people's court on sealing up, distraining and freezing property in civil execution
Article 6 stipulates that the people's court may seal up the residence necessary for the life of the person subjected to execution and his dependents, but may not auction, sell off or pay off debts.
Article 7 stipulates that "after ensuring the minimum living standard of the person subjected to execution and his dependents, the people's court may, upon the application of the applicant, execute the house and daily necessities that are beyond the necessities of life of the person subjected to execution and his dependents."
the marriage law of the people's republic of china
Article 19 A husband and wife may agree that the property acquired during the marriage relationship and the pre-marital property shall be owned by themselves, in whole or in part or in part. The agreement shall be in writing. Where there is no agreement or the agreement is unclear, the provisions of Articles 17 and 18 of this Law shall apply.
The agreement between husband and wife on the property acquired during the marriage relationship and the property before marriage is binding on both parties.
The husband and wife agreed that the property acquired during the existence of the marriage relationship should be owned by each other. If the third party knows the agreement, it should pay off all the property owned by the husband and wife.
Article 41 At the time of divorce, the debts incurred by the husband and wife living together shall be paid jointly. * * * If the same property is not paid off, or the property belongs to each other, it shall be paid off by mutual agreement; If the agreement fails, the people's court shall make a judgment.
Interpretation of Marriage Law II
Article 25 If the divorce agreement of the parties or the judgment, ruling or conciliation statement of the people's court has dealt with the division of husband and wife's property, the creditor still has the right to claim rights from both men and women for the same debt of husband and wife.
If one party claims compensation from the other party based on the divorce agreement or legal documents of the people's court after assuming joint and several liabilities for debt repayment, the people's court shall support it.