I'm not, repeat not, a lawyer, but to be honest I think you may have a real problem getting ownership and that you really do need legal advice. If your parents were prohibited from owning the house when it was bought then I think you will have trouble arguing that your mother can claim any title (but remember, I'm NOT a lawyer). You may though be able to argue that a loan was made to them, and have a chance getting the loan repaid, especially when / if they sell the house or die.
Good luck. And comfort yourself that your parents did something very good and decent for another family, even if it doesn't work out as you expected.