Hiya,
sounds like very good advice the chaps on here have given you. As long as he's not going out and looking for a confrontation then I don't think you have an agression problem at all. I agree that it's not ideal for him to snap at dogs which come over to him, but I've found that Bella can do this to dogs who are over boisterous with her and I'm happy for her to tell them off if they push their luck with her. Some dogs are more comfortable with strange dogs than others, just like some people are better in new social situations than others.
I think yous dog behaviourist has just pidgeon holed your problems into 'sibling issues' because it's an easy diagnosis, I don't think anything that your dogs are showing you points to sibling problems, so get a new behaviourist if you still feel you need one. Definitely try positive rewards in getting him to enjoy meeting new dogs and behaving calmly, if he sees new dogs as a happy, positive thing then hopefully he will not get so anxious about them and calm down without lunging or snapping.
You sound like a wonderful owner, with your dog's best interests at heart
I think it's awful that the rehoming of one of your dogs was put forward as the simplest 'solution'
With these two being your first two dogs and this being the only possible problem with their behaviour that you've encountered so far, I think you've done extremely well (especially with two dogs who are litter mates)
Well done
Be consistent and focus on positive rewards for the desired behaviour and I'm sure he'll improve and gain some confidence with new dogs.
Good luck.
Best wishes
Mary