My little brother is several years younger than me. Growing up, we lived with our shared mother & his father (my stepfather). Because of his young age & father's temperament, my brother got away with any violence he wished to inflict on me, while I was punished severely for any attempt to retaliate.

Eventually, when my brother was a young teen and I was a college student, I had a broken arm in a sling, which due to the location of the break could not be placed in a hard cast. My brother at one point hit me in my broken arm (not particularly hard) and I turned and punched his right shoulder hard enough to knock him to the floor.

He never hit me again.