This is another one of those cases where I don't remember sufficient details to say for certain if this is fantasy or not. It was a young adult book. I'm pretty certain I read it in hardback somewhere in the mid 1990s in the United States as a library book. The main character is a teenage boy from the United States whose family is under the sway of a deadly curse (I believe they keep dying in accidents). He is the last remaining male of his family and so he travels to Scotland to find something related to breaking the curse, possibly the ancestral home. While there, he meets a redheaded teenage girl who seems oddly tied to his quest. At times she seems to be helping him and sometimes she seems to be fighting him.
Eventually, he learns that she's part of a rival family, possibly part of the family curse, and is under orders by her uncle, who's trying to kill the boy. The "curse" is due to a betrayal by ancestors in the two families.
Near the end of the book, they find the place important to his family history, wherein he's seeking a treasure. He learns the truth about the girl, but she decides to side with him. I think there was a treasure they were looking for.
For some reason, I want to say that it had something to do with smoked fish, maybe herring... they find a barrel of it and initially it seems like it's one of those worthless "the treasure never existed except in legend" situations, but it actually turns out to be the key to him acquiring the family fortune, which was in the fish business. Oh, and the girl falls for him although I don't think it went any further than a kiss. This ends the curse, although I don't remember if it was something mystical or a matter of that the uncle is put out of commission and the girl decides she likes him.
The only other detail that sticks in my head is that one of the first times he meets with the girl is after he stays in a local inn as he begins his journey. I've tried searching on the details I remember, but all I seem to get are Romance novels about handsome men in kilts.