Keep the secret that you read the diary, don't say it to anyone, even outside of the family, because if he knows, he could get even more depressive, he will think it's like a betrayal.
Be very nice with your brother, and supportive, make him feel not "worthless".
Tell your mother that you are very worry for him, and show her that you are really nice and supportive for him, maybe she will ask you why you are so nice with him, and you will say again: he is mentally very unwell.
Don't have a conversation with him about that, it's not a good idea. When we feel suicidal and depressive, we don't like that kind of conversation.
Just be supportive. Try to understand and ask him about his life, how he think about things, but don't ask him directly why he is suicidal. Just try to understand him without direct questions.
Be nice and supportive: but be careful that he doesn't guess that you read it, so you have to do it progressively.
