I agree with the other posters who suggested that if you can't keep them safe, a new home for your chickens might be best for them. Often, the hardest thing for us is the best thing for our birds. Have you asked your brother to help with financing a newer, more secure setting for your chickens?
My first thought is chickens were there first, tell him the Springer has to go.
Who's house is it? Yours or your parents? If its your parents, they need to help artbitrate this situation.
That said, as other noted, Springers are bred to hunt birds.
Versus say a lab, which is bred to retrieve or a sheepdog which is bred to herd. Or a pitbull, which is bred to be loyal and protect its pack or hunt larger prey, not birds.
Most Springers will never get along with chickens.
Short of shock collar conditioning, which might work, you either have to secure the chickens or secure the dog or find one or the other a new home.
I have to agree absolutely whole heartedly with carlf. If it's your home, you lay down the rules. Period. If you are living with your parents, they ought to be looking out for the best interests of all the animals they allow to reside at the house. They should also be keeping in mind who was their first, and who will be there long term.
I would never trust a dog like you describe. That high level of prey drive will not go away. You may be able to train the dog to behave in the presence of others, but eventually the dog just won't be able to help itself. Keep in mind that you would only be teaching the dog to override it's prey drive, not eliminate it. That temptation will always be nagging at that Springer. We had Springers when I was a kid. They went after everything! Every other animal we raised HAD to be in super secure housing. Two springers even tore apart a rabbit hutch once. My current dog is a pitbull-mutt mix. She is the chicken poo cleanup crew. She tries to make friends with the chickens and gets scared when they flap and squawk. Breeding of the dog makes a huge difference in their behavior, train-ability, and prey drive.
I think they only option if that dog is going to stay, is to reinforce the coop. I would also make the case that since it was not required before the dog arrived, cost and labor should be shared to get it done. That's just my opinion. Good luck. It's tough when family have conflicting needs/interests/desires. I hope it all works out for you.