wow. tough call.
first off, if no one knows about you, you will have to be prepared for everyone to find out. Once you start outing yourself, no matter how much your nephew might promise to keep it a secret, you ought to consider your closet status as revoked. I'm not saying it will happen, it's just best if you're prepared for the possibility.
Alternately, you can be nice to your nephew, and accepting, without having to say things like "I'm your auntie Jill!" You can always be accepting without outing yourself. This might be the best thing for you, since it doesn't sound like you're really ready to come out yet. If anyone calls you on your acceptance of your nephew, you say that it shouldn't matter. He's family, you love him, everyone else should, too. Your unconditional acceptance could mean a lot to him, and could change the way everyone else sees him, too.
It is always best to combat transphobia with a calm demeanor. If they do start to mouth off and you get offended, don't give in to the urge to talk trash right back. That makes you look bad, and won't win you any points with the people you're trying to convince to be more tolerant and accepting.