Hex in the Highlands by Aurora Grey

She’s a witch who gave up her magic and her chance at love long ago. He’s the warlock wolf shifter who loved her… and this time, he’s not letting her go.
Tamsin MacPherson fled her home in Scotland, and her first love, after her parents died. She built a new life for herself as a doctor in Manhattan and never looked back. But when her sister is turned into a demon by a sinister criminal organization, Tamsin takes her to the Highlands to hide.
Back home at their family estate, Tamsin works furiously to develop a cure for her sister. Facing her ex is the last thing she wants, and a distraction she can’t afford. To make matters worse, seeing him is making the magic she bound long ago come back, more wild and unpredictable than ever.
Blake Blackstock told himself he’d moved on after Tamsin left Scotland two decades before. But the second she walks back into his life, he knows he’s been lying to himself. He’s never loved anyone since her, and he’s going to make sure she regrets her decision all those years ago. And revenge is best served in the bedroom.
Blake has other things to contend with first, however. Danger has followed his love back to their home, and he’ll have to use the full power of his warlock dire wolf clan to protect her. He lost her once, and nothing is going to tear them apart this time.
Because everyone knows it’s pure folly to stand between a wolf and his mate.