The Bitterroots by CJ Box

This is not part of the popular Joe Pickett series by Box. Rather, the "star" is Cassie Dewell, a private investigator and single mom in Montana. She reluctantly gets involved in investigating the case of a man accused of raping his teenage niece. The man is the black sheep son of a powerful ranching family in an isolated part of Montana in the Bitterroot Mountains, hence the title. Blair, the black sheep, had left and found fame and fortune in NYC, which caused a lot of resentment amongst the remaining family members. At first glance, it seems like it's an open and shut case, but the more Cassie investigates--and the more people try to thwart her investigation and threaten her--the more she begins to wonder whether he might not have been set up. At the same time, wildfires are burning throughout the area, which adds to the murky ambiance and sense of constant danger.

I'm not sure whether this is part of a series. The author refers to previous events and cases in Cassie's life that made me think there was at least one earlier book.  But the story holds together on its own.

Addendum: I checked, and yes there are earlier books in the Cassie Dewell series.