A rank for the brothers? Sure.
S rank should be reserved for ships IMHO, and I don't think that HarlanxHarold will be a thing in anyone's mind save for the insane incest-yaoi fangirls
>.> So if someone isn't romantically interested in anyone, they don't get any S rank supports? Me dislike, disapprove, and squint my eyes at thee. Smacks of something-phobia biased against asexual people.
I'm probably reading too much into it, but my point is: it doesn't necessarily have to be romantic shipping, as evidenced by Fire Emblem plenty of times. The one that comes to mind immediately is Ross and Garcia(yes, I know it's A, but they don't have S rank supports). It's just that that's the person you confide in most easily, the person you've grown closest to, your best friend for life.
Will your S rank support probably go with you wherever you go, and you with them, for the rest of your life? Almost certainly. Which in turn means that depending on the character in question, it might actually be inappropriate for their S rank to be their ship, even if they have a ship(they just don't care enough about their significant other, because they aren't like that/into that sort of thing).
But it also depends on the brother's relationship beforehand; they don't seem to be on the best of terms, after all, so while it's nigh-impossible they wouldn't support each other, how high it goes could vary greatly.
Plus there's exactly two girls and one of them is either a lesbian or bisexual, while the other is technically crazy(and quite possibly every sexuality simultaneously because of her Others), so I don't know about you, but there's prolly gonna be quite a bit of Yaoi goin' about if everyone's gonna get shipped.
Then again, there's always the polyamory route.