feeling scared angry and sad makes a lot of sense, if you weren't I'd be worried.
What you're sitting on, and maybe uncovering, is a whole mess of abuse of power, and it doesn't make a whole lot of difference if that abuse is used sexually or violently. And if your young cousin is right then he's suffered both.
The emotional scars can be huge, and proper treatment - therapy - is about the only way we seem to deal with it. But it's often no use in forcing someone into therapy, it's got to be our choice. When we're ready to help ourselves we will.
Your other problem is your brother, and you can only follow your instinct and seek the truth - and I cant begin to think how hard and emotionally dangerous thatwould be for you. Or, you can report him. A hell of a move I know, but the police or other authorites are the best equipped to deal with finding the truth.
I'm afraid you have too many conflicts to be objective, and too much to lose if it all goes to hell.
You're in a terrible position, and the choices must be yours, and I don't say that in any way to make you go away. Far from it, stay around and talk to us, bounce your ideas around, ask what you will.
I hope we can help you through supporting you.
Go confidently in the direction of your dreams! Live the life you've imagined. As you simplify your life, the laws of the universe will be simpler.
Henry David Thoreau