Those people raised your fiancée. I promise you, you are going to find that the girl is just like her mother. Two faced. Maybe you can live with the other head, but maybe the other head will tell you "no more Z car." What will you do then?
It's time for you to give their little girl a test too. It isn't true false, multiple choice, or anything else you've heard of. You don't need your parents to help administer it, you need a lawyer. The test is called a prenuptial agreement, all she has to do is sign. Otherwise, I'm sure she knows where the door is.
Edit: Same advice applies to real marriages.