I grew up around bikes, and first rode on the back of my father's bike when I was 4. My first few boyfriends had bikes, I was 19 before I had a boyfriend who could drive.
DP has bikes. He has been riding bikes for nearly 43 years, including 13 years working as motorcycle courier in London, and many years of daily commuting by bike, as well as riding for pleasure. He had a few spills in his first year or so, nothing worse than grazes and bruises, until a car changed lanes without indicating and hit him, dislocating his hip. That was in 1979. He hasn't had an accident since.
I couldn't begin to think how many miles I must have travelled riding pillion. In all those years and miles, I've only had 3 offs, two of which were injury-free and the third (complete fluke, a patch of spilled diesel at low speed) resulted in a painful, but easily remedied, dislocated shoulder.
I attribute my lack of injury to one thing: never get on the back of a bike ridden by someone who rides like a twat. DP's lack of injury is down to two things: a) he doesn't ride like a twat and b) he has a sixth sense for people riding/driving like twats and gives them a wide berth.
Your brother will have to learn not to be a twat or he will soon end up hurting himself. Hopefully, he will get the non-serious but frightening injury that is a huge wake up call, and either give up or quickly develop the skills that a biker needs to stay in one piece.
Having a serious injury/accident isn't a given, and I'd recommend anyone starting out to take a Bike Safe or similar course once they're on the road.
Protective gear is essential, without it even a small spill can lead to awful injuries. Leathers or really good textile jacket/trousers (ideally including some Kevlar or similar), with protective inserts for knees, shoulders, upper spine etc, gloves with plastic protection for knuckles, top quality protective boots and the best helmet you can find (expect to pay iro £300 for a decent one). The fit of the helmet is critical, so go to a reputable supplier.
It would also be a good idea for your brother to go to some bike meets, like the ones at Box Hill in Surrey, where bikers just turn up and hang out. There are always several with arms in slings, on crutches, in wheelchairs, missing a limb or two. It's very sobering for a newbie biker.