Advice please

So I'm not gay or bi, I mean I had a bi phase in high school but I'm 100% straight now.

However, my brother just came out about feeling like he should be a woman.

Backstory: I was an only child for 5 years. I begged for a baby brother. I finally got one on my 5th birthday. He was bullied a lot in elementary and middle school but I always protected him, I chased a cops kid down the street for picking on my brother. To this day I would confront someone for hurting my brother in anyway. Even though he's going to be 18 this year and it may sound silly.

I can accept his true feelings because he is my baby brother and all I want is for him to be happy. But all I've done tonight is cry one because yeah I'm losing that brother I prayed and begged for. But 2 because the world is cruel, I can't protect him from everything, I'm so fearful of what could happen to him. I know he's an adult he probably doesn't need his big sister still acting as a force of protection, but I can't help it he needed me so much as a little kid.

If you are trans or have a sibling or family member that is or has gone through this please I'll take any advice to ease my fear and worry.