I gave my sister's husband brain damage and nobody knows it was me
Oh my gosh! How did this happen?
I was hiding in the backseat of his car, and I accidentally hit him with the car door.
You should feel really bad for hurting someone. You should admit what you did and help everyone heal from the situation.
I don't want to tell anyone. He won't even get mad if he knows.
If it was just an accident you should tell the truth. He can sue your car insurance company so he can afford the medical bills. If you keep it a secret they might lose their home.