There's nothing you could have done to start this, and there's nothing that you can do to stop it. You can get in the way and make your sibling's life a living hell, you can get out of the way and act like everything's normal, or you can be a pillar of support that your sibling can depend on. This isn't your fault, this isn't your parents' fault; nobody caused this, nobody is to blame, and on the balance, it's a really really good thing that your sibling is questioning this now.
When I was that age, I was miserable. I was depressed, sullen, and withdrawn. I was angry and violent. I was completely detached from all emotions except sadness and anger. That persisted until my mid-30s. I was dependent on marijuana for a large portion of that time; and I spent much of what could have been the best years of my life in a stupor. I didn't know what I was hiding from, or even that I was hiding.
My experience is not uncommon -- I had a wonderful childhood, but when I hit puberty I totally went off the rails and when I turned 18 I found drugs and used them to mask the pain. My parents didn't do anything wrong... this is just part of who I am. It wasn't until my dad died that I finally had to get in touch with my emotions -- I was completely unable to process my grief, and I completely shut down for almost two whole years. As I finally began to examine my feelings, all this weird gender crap sprang up instead of the grief that I expected to be dealing with.
You've got some experience with this... you thought you were gay for a while! How did you come to the conclusion that you're 100% straight? I'm going to guess that there was some questioning; maybe some experimentation... and after living in that identity for some time, you figured out your truth, which you're now living. Your sibling needs to experience this. The question is there... and it can't be answered without putting in that work.
So here's some homework for you. Imagine your family deciding that you're a girl. Like, tomorrow, you wake up and everybody starts calling you by a girl's name, using feminine pronouns to describe you, buying you jewelry and women's clothes for your birthday. How would you react? How would that make you feel? They're persistent -- no matter what you say, they'll keep treating you like a girl! That may very well be how your sibling feels right now.
You can help. Don't tell your parents. Don't tell your sibling that you know, until you think that you're really ready to love them as a brother -- maybe they aren't your brother, and maybe they are... but if you're ready for that then you can be the ally you want to be.
One last thing. Transgender teens are at a high risk of suicide. That risk is greatly diminished if they are supported by their families. Families armed with that information can save their transgender youth by supporting them rather than fighting them. You might save your sibling's life by sharing that information with your parents when the time is right (when they're ready to come out).