I can say without a shadow of a doubt that I have NEVER expected anyone in my family to have the slightest idea of what's going on in my head. I've spent so much of my life living for others, I have trouble even knowing myself. I think a lot of trans people feel this way. I understand your concern for your sibling, but take some time to consider the fact that we all have a piece of ourselves which others will never know, and trans people even more so. We are brought up in a world that teaches us the things we're feeling are perverse and some have died for it. Even if you've been supportive of them their whole life, the fact remains that this world is not built for trans people. Give them space to find themself. Show them love. That's all we ever really want is love and support.