Hi, and welcome!
I too, come from a rather fanatic family. Fortunately, my brother is also atheist. Also, I don't have to live with my parents (fanatic Christians). I know a tiny bit of what you mean though. When I visit my sister, 24/7 I have to censor myself. Most of the censoring is curse words (for my niece and nephews), but the rest of it is hostile responses to religious stuff. It IS very taxing on me, and when I return home I sooooooo enjoy cursing and swearing away, as freely as I please.
I did visit my parents this morning, and they did not try to convert me. I guess after years and years of trying, they have decided it's okay to take a break (from witnessing to me) once in a while.
My sister does actually make an effort to ensure that I am not harassed (witnessed to, etc.) at her home, which does make my stay a lot more enjoyable than it would be otherwise. All that is required of me is to remain silent when she is teaching her children that mankind (and the earth) is only 6000 years old and did exist with the dinosaurs and that there is no global warming, etc. etc.
My 6-year-old nephew occasionally tries to convert me, and his attempts alternately amuse and irritate me, but I know he is trying NOT to witness to me, 'cause my sister has told him not to.
my nephew: If you don't believe in God, that's where you'll go! (referring to hell)
me: (enthusiastically and with great joy) That's RIGHT.
Not so amusing:
my nephew: Are you a Christian yet?
me: (biting my tongue HARD) You already know the answer to that. (what I wanted to say was "Are you an atheist yet?")
I do keep reminding myself that he IS only six years old and does not want me to burn in hell for all eternity. But every time I remind myself of that, I become more annoyed at Christianity itself and how it threw hell into the mix, to obligate its followers to harass the non-believers; at least the non-believers that they don't want to see end up in hell.
Another not-so-amusing exchange:
me: Wow, that was lucky.
my nephew: There IS no such thing as luck (implying that there are only blessings from God)
me: (biting my tongue and saying nothing. What I wanted to say was "there IS no such thing as God").
Perhaps you can find a not-so-fanatic Mormon to be friends with; one who does not feel the need to convert you. My favourite real-life stamper friend is actually a devout Catholic, but she never ever tries to convert me or ANYTHING. I actually did not even realize that she was religious until 6 months into our friendship!!