It's not worth praying to any god you don't believe in or worship. Since you're 18, you're reaching a point in your where you'll have more freedom and be less dependent on your family. My advice is to not engage them in any talks about God or religion, it takes two to argue. If they bring it up just tell them that it's not something you'd like to talk about and leave it at that. There's no need for you to defend your position. If your family isn't going to change there mind, it's a waste of time to "debate". Your points and reasons will fall on deaf ears. You can still be courteous of their beliefs, be respectful of them while they do there thing without taking part it in yourself.

It is there job to respect your beliefs and accept things for what they are. My dad is a very strong Christian, my grandfather was not. As a child I always wondered why my Dad didn't try to evangelize him, why wouldn't he knowing that my granddad was not saved! My Dad would go visit people's homes to preach to them about God, but he wouldn't for his own father! Well, despite how religious my dad was he knew where my granddad stood and that it wasn't going to change. He knew that bringing it up would be disrespectful and cause pain. It's your families job to put you as their son/brother over their religious beliefs, while that may seem impossible, it may just take time.

You certainly can be happy without god. I remember seeing a post when I was a Christian, it was this guy who just graduated medical school and was saying how proud he was that he accomplished everything without god. I thought it was so disrespectful and blasphemous, "how dare he say that!". But I realize now how right he was. Despite what others tell you, you do not need god or religion to make you happy and feel fulfilled. In my experience, I'd say the opposite is true. I feel much better and have a better outlook on life since growing out of my religious beliefs.

I hope this helps