Lingering glances, fleeting smiles and the briefest brushings of hands in bustling corridors was the extent of their platonic relationship of sorts. (It could have been likened to a courtship as it was in most ways.)
It wasn't exactly a secret (his father – her youngest brother – knew, or at least, suspected), but it was hardly well-known - or quite acceptable.
He was married – and any who knew of his wife, or had perhaps just glimpsed her in passing, would wonder why he'd be willing to give up her for some 'washed-out' older sister of the Lord of the skies.
And she'd never really had a proper relationship. She may as well have vowed never to marry for all the good that'd done her, the idea of marriage never even had appealed to her – until he'd wormed his way into her life.
But by the time she'd fully realised that she reciprocated his feelings for her it had been too late. He'd already married.
I shall leave you to decide just which pairing(s) I've implied.