I was worried when my parents found out I was a atheist too, but they are far more laissez faire than your mother. My mom is true non-denominational christian and my father is mostly too, but with a dose of catholicism. They have no dogmatic beliefs, which is nice.
On to the advice. Be supportive of your sister. Tell her that her choices are always her own, that your mother is to be respected, but she is no supreme authority. Your mother needs to know what you believe. You don't go out and have a orgy every night, you're not some amoral psychopath. You don't believe in nothing, you believe in people. I would recommending calling yourself a humanist and/or a naturalist, meaning you believe in humanity as a species, community, society and the natural, observable world because these words say something about you. Calling yourself an atheist says very little about you. It only states that you don't believe in God, not what you actually believe in. It probably won't help much, but it certainly can't hurt. As for your extended family, you either continue to live the lie, which is painful, or you get it out of the way and tell them that you're going to make your own way while explaining what you actually believe, this will hurt too and they will probably ostracize you or spend every minute with you trying to convert you. You are between a rock and a hard place with the extended family issue. In the end, you can only do what you feel is right. As for your other problems, I haven't the personal knowledge to help or even to offer a solution.