Re: Incarnate Angels and "siblings"
I've always found the Sibling thing a bit amusing when I see it with the YHWHic Angelics, it just always hit me as cheesy, that someone happens to be ArchAngel Michael's twin brother who was never mentioned anywhere, despite being as awesome, powerful and cool. Obviously, that's the more extreme end of the belief, but the general undertone is there.
I think you're right that it may deal with the Choir/Orders, that people may resonate with someone of the same Choir/Order as them, the most well known are the big-wigs (big-wings?), and rather than assume "We're similar because we're both X" they think "We're similar because we're brother/sister" and the phrasing is always used intimately.
To contrast it with Rakshasa. Yes, I call every Rakshasa Brother (except Miniar, for being a half-breed :-p), so while that includes the most well known of us, like Ravana and Vibhishana, it also refers to every single one of us. We consider ourselves Brothers (technically Siblings, but Siblings is too removed), because we all come from the same Mother, but when I say Brother is doesn't have the emotional connotation that it is being used in our culture, in most of the Angelic cases above. There were hundreds of us, the amount of "real" connections were fairly small. While I refer to them all as Brothers, honestly there are less than six, that I feel I know/remember.
Time for the but....but...there is a strong resonance between us, and I can see a Rakshasa who doesn't have context for the sensation thinking it is something personal, rather than impersonal.
I think that's what happens with a lot of Angelics (assuming they aren't just trying to make themselves sound cool), they feel these connections, and have no context for them, so fabricate (consciously or unconsciously) a set of memories to explain them, and not knowing the nature of the resonance assuming it has to be something deep, emotional and personal, so brothers/sisters/twin/split soul, rather than same Choir.
When we first begin all things simply are.
As we grow all things are external.
As we learn all things are internal.
As we understand all things are not.