The Mermaid of Zennor

Lilac misses Jesse so intensely she can think of nothing else. As the days pass, she becomes more and more fixated upon him, until her obsession threatens to consume her.

When she learns that her brother will be coming back home for Christmas, at long last, she can't contain her excitement...

But her brother, as she soon discovers, is not alone. A strange woman has ensnared his heart in his absence, and Lilac finds her already fragile mental state unraveling faster still. She knows she must free her beloved brother from the grip of this wicked enchantress, no matter the cost, if she is ever to be happy again - but just how far is she willing to go...?

[From Steam]