Oldest daughter(s) relentlessly tease their little brother? Teasing 101?

What you are observing is bullying and harment. You are right in thinking it may permanently damage their relationship. You are the parent, and it is your job to put a stop to it now. Being an oldest myself, I know little brothers often deserve the aggression they get because of how they instigate. But it's damaging to all of them to let it continue. Next time they get going like that, make them all wash windows or mow the lawn or clean the bathrooms. Harment isn't so much fun if you know it will earn you extra work.