After your mother's death her estate may be liable but you as heirs or beneficiaries would not be required to accept any liability.
You should consult with an estate planning atorney that may be able to isolate you mother's other assets from liqability for this debt.
While living your mother remains liable, so risk that the timeshare organization could pursue, but the attorney can evaluate risk including other assets and protections from judgment debt under federal and/or wisconsin law.
An attorney needs all facts and documents to review. Your mother's situation is all too common with timeshares, especially if they are not successful, no market to relieve your mother of obligations.