Sister using us as a co-signer?

My sister in law is 19 and going to college. Her Mom is trying to get my husband and I to co-sign a loan for her because the parents and all 3 siblings have bad credit. I asked why they can't get financial aid and they said they will only give her 5,000. This loan that we would co-sign would be for 15,000. The Mom is actually telling my husband that we are his sisters only chance to go to school. What would you do? I am pregnant for the first time and we are already have loans out from school, our house and cars. I just don't want to get bad credit and have to take this loan over in the end. Any suggestions or stories that relate?