I'm assuming there is no will, no mention has been made of one;
Aha, I forgot that civil inheritance law is different to common law.
Okay, to try and explain - Polish law works on the principle that an inheritance can either be accepted or denied. If someone dies without leaving a will, then there is a formula as to who inherits what. Now, what matters here is that debts can also be inherited, not only assets.
From what you've said, it's probable that he's taken some sort of financing on the car. So - she wants you to agree to deny the inheritance, so that she can sell the car and pay the financing costs. The car is part of the liabilities of the estate, so if your daughters accept the inheritance, then they have to pay for half the car as the finance agreement will be terminated.
The question is: do you think he's got any assets? If you have any reasonable doubt to believe that he does, then you should enquire with the court that's processing the inheritance.
CMS - it's only 100,000zł, you can get leasing deals for that without much trouble at all. So it might be above board, especially if the wife also signed for it.