I don't think has to do much about fashion modesty but more what the brothers are a custom to... i assume the brothers are used to only wearing the old big style suits and i presume they see anything out of the normal wrong.
i personally don't see much wrong wearing either suits is like wearing either a skinny tie or wide old style tie.
I have both types of suits, but i think skinny suits can be modest too as long as they are not too extreme.
Anyways my honest opinion.