The hardest part of explaining trans stuff to kids is their parents, so if your sisters are already supportive, explaining it to the kiddos shouldn't be too hard.
With kids, the younger they are, the more flexible they're likely to be with new concepts. I recommend setting up a time to tell them, like "I have some news for you guys, so let's go out to dinner and talk!" Make it sound exciting, not grim. Your approach/mood will totally impact how they react to the news. When you get there, just tell them-straight forward that you're actually a boy (along with the basics, like what you want to be called). Then, for the more detailed stuff, ask if they have any questions and then answer their questions. If they ask anything too personal, either give a vague answer or tell them it's private... but do try and answer as many as you can. Kids are curious creatures. They may seem like they're being nosy, but really they're just trying to learn more about the world. The more open and honest you are with them, the more likely they are to trust you, and the more they trust you, the more likely they are to respect you and accept what you're telling them.
If you're feeling especially ambitious, you could even do something like having a little party (almost like a small birthday party) for your new name/gender with balloons and a "Welcome to being a boy, Uncle Gene!" cake or something equally fun and silly (maybe even get the kids in on planning it), to help drill home that changing identities are something to celebrate, not bad news.
