I told my sister I'm bi but I might be gay. Have I messed up in telling my sister something that could not be true?

Firstly, well done you for reaching out. I think your sister would be impressed by your capacity for vulnerability with her. It isn't easy to open up to people, especially people we are close to whose opinions we hold on to so highly. Take your time to figure out how you feel, there's no rush. This may even be an open dialogue that you have with her, were you can collaboratively figure it out. At the end of the day she's your sister and she loves you. There is no messing up. With any luck, you will have a great ally in her. All the best friend :)

No not at all! You have not messed up! It is totally okay because you are still discovering yourself and what you like. There is no reason to feel ashamed or bad. If anything it is good that you are able to figure out what kind of things you truly like. Sometimes people do not realize what they actually like until later on in their life. It is great that you are able to tell your sister this as well. Your sister would be happy that you are figuring yourself out and that you are comfortable enough to share your thoughts and feelings with her.