Wow!! Definitely find another vet. I've heard some pretty stupid/arrogant things from some vets but this one beats them all. Size does not equal reproductive ability. Regardless of how big or how tiny the dog is does not matter. Where there is a will there is a way and you can bet those other boys are going to try. What is even worse is that a larger dog could seriously hurt her or kill her just trying to mate with her, they don't care how little she is. Does he think that a male dog would think to itself "she's too little to mate so I won't even try."
Heck no!! Any dog that is in heat they will breed to.
As for the brother that doesn't want to neuter his dogs, this is so stupid. Male humans are way too attached to their testicles.
Does he really think the dog is going to care or sit around depressed because he no longer has them? Hate to break the news to him but they don't care and in fact neutered males are actually much happier than intact ones as they don't have all that added stress anymore so they aren't trying to hump everything, pee on everything, mate everything and run away to mate everything.
I'm glad your friend has you by her side, you probably just saved that little girl's life!!