It sounds like you may not be on board with the GSD? This will be your brother's dog?
Since everyone's a boy, it should be ok. Don't worry about dominance. Your pup and his pup are both close in age which could be an issue, but if one or both are neutered before they are 2, chances are you aren't going to get into the major dog-dog issues.
Training is key with all dogs. I am not sure what your brother is planning to do. But what you can do is have such a fun time taking your dogs to classes and training them, that he will want to do the same with his boy.
Lots of people have GSDs with other dogs with no problems.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC