A bit random, but this "saint" Hermie Schultz that was introduced at the end has a face very similar to Schnee's.
Might just be a quirk of the art style, but I wonder if there's a connection there.
That 'congratulations' might be the first genuinely positive interaction we've seen between Carlo and the Duke. When Carlo initially left for the academy, it was just a terse farewell.
@captainunderpants123
It's because it's a very easy target for people who don't like the Castiellos. They mentioned earlier in the story that the Castiello family always has red eyes and black hair, so singling her out as an abnormality is an attempt to get under her skin and embarrass/belittle...