I would just say I was an atheist if the subject came up, refuse to do anything I wasn't comfortable with (like prayer). If they ask you why, or say anything, simply question them as to why they believe what they believe, and simply respond in a calm manor why their point is not valid.
If you unsure on how to answer something there are many youtube videos for many responses to many questions. In the end there is an atheist response to everything they throw at you. You don't have to be aggressive (with family I wouldn't recommend it).
In my experience these things are a bit heated at first, but they get used to idea pretty fast (especially when none of their arguments hold water) and sometimes (albeit a rarity) they may even come over to your way of thinking. Personally I sometimes struggle with getting overly angry and it doesn't get me anywhere. Be calm, be cool, and make sure not to be the first to attack, you'll always have the moral high ground.
As a side note, I'm not sure if it bothers you when they say they'll prey for you, but ask them how they would feel if a Muslim said the same to them. Personally I find it condescending and insulting.
Its so easy to feel defensive when the people you're talking to are ganged up and talking about 'saving you'. Maybe it would best to get each one (the important ones at least) and say what you want to say alone, rather than in a group.
I've debated chistians since I was 13 (its sort of a hobbie) and I find the best thing has been to stick with what you know (which is why watching other people debates is so useful). There is never any shame in saying 'I dont know' to questions like where life and the universe came from. It is enough to say not knowing something does not indicate the work of the divine (hell I dont know how my tv works, but I dont think there is a little man in it).
Also I would probably avoid the thinking that 'god had to come from somewhere' that is the thinking they use when they maintain a creator must exist (it seems to me entirely possible that the universe has been around forever), but it is a good point if they say 'everything must be created'. If you can look on wikipeida or something to find what scientists really understand to be the case, it will help you alot more than making assumptions on what 'seems to be the case' which is what they are doing anyway.
As far as your wife goes, if you are able to live a happy life with her I dont see any issue with her being a believer (personally it would drive me crazy, I would always rather someone had the respect for me to make sure I knew what was true) but not everyone thinks like that, your happiness is important, dont make your self miserable. I'm sure your wife will love you enough to love you no matter what you believe if you can do the same for her.
May I ask what denomination of christianity your wife and her family are?
Oh, I know, you wouldnt believe the lies some of them convince them selves of, the import thing is to answer their questions as honestly as you can. (I'm sure we can help you out with any arguments you cant give an answer to) but again, they think if we dont know something about our world it proves the existence of god, try and explain to them 'the god of the gaps'.
And always say 'I will believe when I see overwhelming evidence'. I've never managed for anyone to give me any (or even underwhelming evidence).
god of the gaps is where the christian will take the holes we have in our knowlage of the world (where life started for example) and they will say becouse we don't know that it only makes sense that it was a god (in their case they are of coarse only talking about the christain god).
so as our knowledge grows larger and larger their god grows smaller and smaller, once upon a time lightening was attributed to gods, as it was a gap in our understanding of the world, now that we understand more, those gods have meant less and less.
so god of the gaps just means they are using what we are still ignorant of as evidence, the phrase will often be "you dont know, but we know it was god"
I would very much recommend any debates on youtube, or books by Christopher Hitchens, Sam Harris, or Richard Dawkins.
Also, you never 'disproved religion' which cannot be done, there is simply no evidence for it. Just as their is no evidence for islam, or Norse gods, or unicorns. I understand that many christians will never turn, they are built that way, but that shouldnt discourage you, I have had experences where people see the logic in what you say, and it is a good feeling indeed.
Its easy to be neutral towards them until you see by looking through history what religion has done to people. "good men do good things, bad men do bad things, and it takes religion to make good men to do bad things".
Its rather amusing if you read the bible, god is a far more distuctive, spiteful, hateful, murderous creator than the devil ever was.
I think its really impressive that in that environment you have had the guts to do this, its been easy for me as my father is very anti-religion (though he never pushed that on me) so congratulations for breaking free :)