Honestly... I know that you won't allow a dog to get neurotic or out of hand. And seeing you with Katalin probably makes him want a dog to be his pal. I don't think there's anything wrong with that, and I do feel that your current situation is the ideal one to ensure that the dog gets properly raised, trained and cared for while your brother learns the ropes. I was very immature and irresponsible when I first got Gonzo, and he totally changed me for the better.
I'm partial... but I do think a Pembroke or Cardigan Corgi from a really good show breeder could fit super well!!! I prefer boy Corgs, but girls are amazing too, and usually super sweet and submissive. They're active enough to do all kinds of sports, activities, and chill enough to be the best cuddlers at the end of the day. They also love playing with bigger dogs and are perfectly capable of sticking up for themselves.