Your sister cannot be detained or jailed dahil sa utang, kasi civil case lang ang pwedeng maifile sa kanya. Criminal case dapat yung nature ng kaso para makulong sya, according sa batas natin.
That being said, she has to pay her debts in accordance with the agreed-upon interest rate. It is possible to contest the validity of the interest rate kung masyadong mataas. Pero sa korte lang magagawa yun and yung judge yung magdedesisyon kung masyadong mataas yung interest rate or not.
You may want to look into the Insolvency Law in the Philippines, since this seems to be the applicable law for your sister. Try to get free legal advice from the Public Attorney's Office nearest to you para makakuha kayo ng bird's-eye view on what you and your sister should do to fix those debts.
https://ph.answers.yahoo.com/question/index?qid=20100615073942AAxdQUf
The above link is similar to your current situation, and it also has a good answer as well.