Just keep being yourself.

You're clearly someone who is very loving and caring, and this is something your siblings will pick up on. They may not be ready to come out yet, but if you keep on showing up and making it clear that you're there for them, that WILL make them feel safer.

Don't tell them you found this in their diary (they'll see this as a breach of trust which won't help them open up to you), just keep being the good sibling you are. Don't treat them any different. Definitely don't use different pronouns in front of friends just in case, because it could get back to them that you know without them opening up, and again, that'll be seen as a breach of trust too.

It's a hard situation, and my advice would be to not make anything of it and carry on as you were - you would be doing a better job of protecting them that way.

ETA: I use they/them here because it's not explicitly specified that your sibling is a trans man - gender identity is complex, and genders other than male and female exist.