"I'm sure the car is fine" makes no sense
Movies cheat so much. Agents can't use the ability to take over people's bodies in 'The Matrix (1999)' before the audiences are revealed that ability, and this movie can't acknowledge that the family has AT LEAST two cars before the audience is revealed that fact.
Why can Marty have his own car, but the other family members can't? Why would they even live in the same house, if their lives are so different? Wouldn't Dave that always wears a suit to the office at least live somewhere in the city area or at least his own house or apartment?
In any case, WHY would the family assume they talk about the BMW when Marty talks about "the car"?
Why wouldn't at least SOMEONE say something about Marty's OWN damn 4-by-4 car? "So what if THE car is wrecked, you have your OWN car?" would be more logical than "why am I always the last to know about these things", or whatever.
The whole family reaction about 'THE CAR' just doesn't make any sense, when we take into account that Marty has his own car, and the family should be aware of at least TWO CARS, so even if "THE CAR" (and it doesn't make sense to talk about 'THE car' in a family that has more than one anyway!) is wrecked, there's still "A CAR" that's not.
This family's reaction to what Marty says makes no sense.