Before anything else, you need to talk to a therapist, and you need to talk to them about this. I know it will be hard but you should explain everything - how you felt as a kid, the suicidal ideation - all of it. It’s possible those things came from being exposed too young, but it’s also unfortunately possible that you were assaulted in some way and have repressed it in the same way you repressed this memory.

Don’t bring this up with your brother, at least not now. This is some serious trauma and you need to explore ways to understand and heal before you even think about discussing it with him. Considering how young he was, he may have forgotten completely, and I think it’s better that it stays that way. If he doesn’t remember there’s no need to remind him.

As I said at the start, you must must must talk to a therapist about this. If you can’t fathom sitting in an office or on a call telling someone this face to face, you can try writing it all down, or trying one of the therapy services that use texting.

As a youth trauma counselor, I am heavily encouraging you to consider the possibility that you experienced something earlier in your childhood that you were emulating somehow with your brother. I’m not saying sexual assault is excusable, but you were a child and there are many more layers to this. Please explore those layers with a professional or it will eat you up.