How do you respond to it might be God's plan?

i have family members who seldom go to church but they know that church is very important to everyone else in the family. whenever they do something that i dont think is right and i try to persuade them not to. i try to steer them down the right path but, they always use the line: "how do you know that it is NOT God's plan?" i never know how to respond to this because i dont want to go into the whole: i doubt that you have even asked Him! ordeal. its extremely frustrating and i dont want to push them away. what would you do?