Night of the Highland Dragon (Highland Dragons) - Isabel Cooper
That was a v. enjoyable read. And...unusual. Why? Well, the time period for one. It's set in 1898. The h is dubiously considering having the castle wired for electricity. Most historicals are vaguely regency (some more than others, presumably because authors can't be bothered to research the time period). The other unusuality is that the *h* is a dragon, and the H merely a suspicious human who happens to work for an agency aware of "other" things.

The plot is simple - a dead person sought out someone having a seance and clued them in to his being ritually sacrificed. The H is sent to investigate. Comments from locals at a nearby town suggest the lady who lives at the castle never ages, and the village is...odd. The lady in question's meeting with him sets off warning bells with both of them - she, because she doesn't believe a word he says (well, I guess if you're a nearly 200 year old dragon, you've probably learned to sniff out deceit), and he, because what he'd already heard coupled with well, something was off about her.

Eventually they do manage to figure things out, both personally and professionally. It is...amusing really...when he does finally learn her secret. I'm not sure what he thought she was prior to that as we're not privy to his inner speculation but he had to have been a little suspicious after the alley fight.

There was one line that had me a bit baffled. I'd have to get the first two and read them but I don't recall another sister.