Welcome to the forum Tj. Are you only 99.9% sure because of your family? I mean, if you KNEW your family would be supportive, would that be enough for you to just be who you know you are? The only person that can really tell you if you're trans, is you.
I agree with the others, you need to try and find a therapist close by that you can talk to. I know where you're coming from having a very conservative, religious family. I came from the same background, and I'm extremely grateful for having very supportive parents, and a brother who always thought of me as a brother. If you feel your brother will accept you, I think you should tell him first. talk to him, one on one. Let him know your worries about the rest of the familiy, and let him know you don't want him to tell anyone else if you're not comfortable with that.
There are still aunts and uncles who "love me" I'm sure, but they don't fully understand, even if they do "accept because everyone else does". I simply, don't keep in contact with them. I love them, they are my family, but I'm not going to go out of my way for people who can't come to terms with who I am.
I'm very close to one aunt, who still to this day asks me STUPID questions. Like "So are you still planning on having surgery", I know she asks just because she doesn't udnerstand completely, that's fine. Even though some of her questions, make me want to slap her across the face, because we've had these conversations over and over again, she always tells me "I love you no matter what you decide in life". And I guess the fact her best friend is a lesbian, makes it easier for me to excuse her ignorance at times.
Everyone will react differently to you telling them who you are. And no matter what the outcome, you WILL feel a lot more comfortable with being yourself once it's out. They whisper talk because they have no knowledge. People are scared of the unknown.