See A Highlander by Rebecca Preston

When Annie Mills embarks on a ghost-hunting trip to the Isle of Skye, she doesn’t expect her biggest challenge to be her cousin—and nemesis—Bella Grantham. But a heated argument in the ancient ruins of Dunscaith Castle sends them both tumbling through a magical doorway into 16th-century Scotland…

Now stranded in the past, Annie finds herself in the care of Tom MacLeod, a calm and capable herbalist with a kind heart and a quiet strength that immediately draws her in. But as Annie adjusts to life in a time of swords and superstition, Bella’s selfishness and schemes threaten to turn their already difficult situation into a disaster.
As Annie dives into the mystery of Dunscaith’s ghostly builder, the legendary Scathach, she and Tom grow closer, building a partnership that balances her modern curiosity with his steadfast Highland wisdom. But Bella’s relentless jealousy—and her determination to take what Annie holds dear—brings danger to them all.