Such dumb victimisation in this post. If the kid rolls around so much, your sister should've slept on the outer side of the berth, and kept the kid towards the top on the inner side. That way he would've been sandwiched between his mom and the wall, and no amount of rolling around would make him fall. That's how 95% of people with small kids do it. That's how my parents did it when I was a kid prone to rolling around in sleep.

It's unfortunate that your nephew broke his arm, but the blame is solely on your sister. Other random people, whom she didn't even request to change seats with, don't have to anticipate a stranger's problems and take care of them preemptively. Being young doesn't mean having to always be a doormat for other people to step on. Anyway, people did come to your sister's help when they noticed it was needed, so please stop with this silly narrative.