So I'm going to try to take a guess and see if I'm seeing this situation correctly...
I'm assuming you have a certain "style" with your photography that your family members like, and they are trying to parlay that into their wedding. Whereas you cannot parlay your style into a wedding photography format, especially at the "sweet" deal they are looking to get out of you playing the "family" card.
In this case, if you don't want to default to the often brash, but true, tactic of telling them the base truth (you don't do wedding photography) I'll suggest this.
Sit down and tell them that your work product for wedding photography will be much different than your normal style. And if they are willing to take a chance that they will like it then you can proceed to shoot and edit the photos however you feel best.
To use a sports analogy, it's similar to asking a veteran catcher to play center field. Sure he knows how to run, throw and catch just as any center fielder, but there are nuisances that the catcher just doesn't have mastered as a veteran center field does. But if the manager is willing to live with them, then "play ball!"
Love that analogy!