I do not like Sirius very much in this fic - but you portrayed him as the perfect cad. I can't help wondering how Harry thought of him after the fact - going out of his way to get Harry's "sister" compromised, instead of defending "her" honor as a proper friend should have.
Severus and Harry were, as pointed out by Centaury Squill above, right out of Georgette Heyer. Both of their lack of the proper social skills was amusing, but appropriate. I especially liked that Severus was somewhat evil - and a little sad to think that the only way he could acquire a wife was to place her in disgrace and force her into it to avoid scandal.
Draco was an absolutely delightful character; I have never seen a fan used as both a discrete form of communication and a weapon before. And leading on an always clueless Ron...