I'm going to give you my unvarnished opinion because you asked for it.
Telling you to "keep quiet" about your gender identity is cruel and controlling. We have a right to be who we are and to let others know.
However, just as they shouldn't be controlling what you say, you can't control what they do. If they're going to "tear their hair out" over your sister's knowledge that her sibling is trans, you can't stop them. That's how they'll react.
Very young children (3-5, say), will understand better by what they see. If they see you presenting as a male, they will accept that. If they ask questions ("why do you look like a boy") you can answer something like "I'm a boy inside so I don't like looking like a girl."
Older kids (6-10), can understand that you're feeling something. For your cousins, you can say "I feel like a boy inside so I'm trying to look like a boy. I like when people say "he" and "him" or "she" and "her" when they talk about me."
As for your sister, she will understand if you tell her "I am transgender, which means I was born a girl, but inside I feel like a boy, so looking and acting like a girl makes me unhappy. So you'll see me look and act more like a boy in the future. I like when people say "him" and "he" instead of "her" and "she" when they talk about me. I might even do things that turn my body into a boy's body. But I'll still be the same person inside, and all always love you and be there for you."
Good luck. I hope this works.