Is your family and best friend coming to dinner collective noun and verb disagree?

In this case it actually doesn't matter that one of the nouns is a collective noun. Since the question inquires about more than one person, the correct verb form would be "are."

Other similar examples:

Are your brother and sister coming to dinner?

Are your neighbor and his children coming to dinner?

Are your coach and soccer team coming to dinner?

A handy way to test for correct grammar is to turn the question into a statement:

Your family and best friend are coming to dinner.

[and not]

Your family and best friend is coming to dinner. (wrong!)

Another useful test is to use a pronoun to reduce the sentence to its simplest form:

Are they coming to dinner?

[and not]

Is they coming to dinner? (wrong!)