Forced Desires series
It started so short and so heavy but became a whole series of shorts/novelettes.
FD 1 is between Katie and her stepbrother Carter.
FD 2 is between Katie and her stepbrothers Carter and Brad
And then, we move to Forced Desires 3. Katie, Carter, Brad and Henry. Her stepdad.
So yes, it's full on stepsiblings/stepdad trope and begins with NC and develops into CNC
You can find the entire series on Amazon.