First even to sue ( or at least to win) there has to be some proof of fraud or some idea that was unrealistic.
First it appears you are not sure about how many loans, what type of loan or loans there was, interest rates and even the amount now due and what makes up
This is the first real issue to sit down with your folks and honestly talk about everything. It is very possible there is more to this than they have mentioned.
If the loan manager misrepresented the loan you may have a case of fraud, which may also have criminal charges.
But of course most of this should have been brought up by your parents attorney at the foreclosure hearing.
The real issue is proving that they did not know what the amount of the loan was, what the payments was and what the ballon amount was and when it would be due, Since all of this should be clear and visable on the loan paperwork that was signed.