I believe I read this book in the late 1980’s or early 1990’s.  I also believe it was part of a series with the two main characters.  This may have been the first or second book in the series.  I do not think subsequent books took place in Scotland.  The two main characters were a guy and girl (possibly a couple, I do not think they were related) in their late teens or early 20’s.  There may have been a romantic aspect of them getting together.  I remember the guy being self-assured and flirty and the girl being more sensible.  In the particular book I’m remembering, they travel to Scotland and stay in a castle near a loch to solve a mystery.