So glad I found this site!  I need some help.....  I have 2 GSP males.  Both are 1 1/2 years old.  they are brothers from different litters (same father).  One (our 'rockne') is becoming agressive with the other.  There's been some urinating in the house (which has calmed down).  Now however, the one loses his mind at 2:30 in the am as they sleep on the floor at the end of our bed.  He wants to fight bad!  The lights come on, I yell at him- and grab his collar.  He growls at me for a second- then figures out that its time to calm down.  At first I thought he was dreaming- but 2 nights in a row???  Now this am, he nipped at my 10 yr old as my son tried to get him into his kennel for the day.  I should preface this by saying that both dogs are normally very well behaved.  The agressive one may be a little spoiled.  Neither are neutered.  Neither sleep in my bed.  They are sometimes allowed up on furniture.  They have 5 acres to roam when we are home.  Thanks for the tips/help ahead of time!

Yes I agree with having them both neutered.  But please don't believe that will be an instant fix.  He is already trying to be the one in control and the homones from being intact do not go away instantly once they are neutered.

And for kids to grab a dogs collar, well I will just tell them never do that.  You need to play some crate games with them using treats and/or toys.  Search that on youtube.  It is something your kids can work with the dogs as it will never inolve grabbing a dogs collar or trying to shove them in.  I am a true believer in making sure a dog is always excited about going in their crates.  Just makes life so much easier.

It seems you have a dog that is trying to be dominant (even with the humans in the house) and the only real fix to that is consistent obedience training.  This can be challenging if you have children.  I would certainly contact the breeder about this and get his/her input, if nothing else, to put them on notice of their breedings temperment.

 I  am certainly not an expert and have had similar situation, but with our larger dog turning our smaller dog into prey, or...just fighting, who knows.  I am currently in obedience training with our two GSP's, male/female littermates, who are 13 months old.  I have learned a tremendous amount in just 2 weeks.  I have learned that every dog is different, and must be trained differently, however, for dogs, like my male, who like to assert themselves as pack leader, you have to train hard.  We have separated the two pups, and EVERYTHING that Kaido does, he does because we allow him, or command him to do it. When he is told to sit, he doesn't move until given the "ok."  When he is told to kennel, he doesn't come out until we give the "here" command.  He doesn't get on furniture or beds, and he doesn't eat without working for it first.  His lessons are hard, and he is given no slack, but after a few pouts and pushing past his stubborness and winning, he is getting the idea!   He actually seems to be a lot more relaxed.   These dogs with aggressive tendencies have to believe that there is a consequence for not following the command, and even more, that they will not like it.   When you have aggression, or even a tendency towards aggression, (when you give an inch they will take a mile or more if they think they can get away with it,)  you have to be tough.   My breeder/trainer has shown me that what I thought was sadness coming from my male, was truly obstinate pouting.  He has come full circle from where he was just 2 weeks ago!  As a test from time to time,  I can command him "outside" in the morning,  and he'll run out super , super excited, (been in the kennel all night,) only to be called back with  the "here" command.  He has enough respect for me now that he knows he better come back immediately.  I have not introduce any e-collars yet!    My breeder/trainer told me that in most cases these dogs do not really want to be the leader, but if no one takes the role, someone has to.....at least in the doggie world. It's what they understand.    Their universe doesn't exist without a strong pack leader.   It's somewhat sad to not have him cuddle with me in my lap, or jump on the bed for a good cuddle, but right now, it's a "professional business" relationship only!    I cannot afford to let my emotions get the best of me, because in the end, it will be better for everyone.   After awhile, I will have earned enough respect to "invite" him for a cuddle on the couch, but for now.....no can do!       Good luck, and like everyone said, be careful.