My younger half-brother used to hit me all the time, and my parents basically just told me to ignore it because he was younger / we lived with his father (my stepfather) who seemingly always took his son's side.

As a young adult, I had a broken arm which (due to the location of the break) could not be cast and instead was just sitting in a sling.

My little brother hit me in my broken arm (not particularly hard, but still), so I turned and punched him with my good arm so hard he fell on the floor.

He never hit me again.