You didn't ask me, and I'm sure you're getting lots of good advice already, but for what it's worth: Take your nephew in, tell your sister that you're not going to turn your nephew away, and remind her that her son—who was expelled from his evangelical high school four months before graduation—is eighteen years old and an adult who can make his own choices about where he wants to live, with whom, and whether or not he wants to go to "a Tampa church that can 'cure' him."
Man, I wish my mother was still alive—she really came through at moments like this. Fifteen years ago a high school kid I knew came out to his parents and they did not take it well. He grew up in a rural part of Washington but was going to a school in Seattle with an arts program where he boarded. His parents were threatening to pull him out of school and drag him home because it was Seattle and the fags he'd met here that had made him gay. I got his mother's phone number and passed it to my mother. She called and told this kid's mom that she was doing everything right—if she wanted her kid to run away from home, that is, live on the streets, engage in survival prostitution like so many other homeless gay teenagers, contract HIV, and be dead by 20. I can hear my mother's voice: "If that's what you want, you just keep it up—you're doing everything right."
This kid's parents came around, our moms became friends. If it were possible, Kos Diarist, I'd sic my mom on your sister.