The Wisp and the Wolf by Jeanne Renee

Meg Quinn has a promise to keep. A talented but withdrawn artist, she begrudgingly travels to the wilds of North Wales to fulfill her late mother’s dying wish—to make amends with her estranged and pregnant identical twin with whom she shares a clairvoyant bond. But before Meg can reach her sister, an inexplicable encounter with an eldritch yew tree thrusts her deep into the past.
The year is now 1402 and the native people of Wales, the Cymry, are locked in an ambitious uprising against England. Here Meg meets Steffan Blaidd, a war-weary Welsh archer who finds himself outlawed by a powerful rival and forced to join the revolt after saving Meg’s life—a revolt Meg knows to be doomed. Plunged into the harsh, vibrant world of the Cymry, Meg knows the mystical connection with her sister demands resolution—as does her growing affection for Steffan and the world to which he belongs.

Torn between desire and duty, the future and the past, Meg must discover her own strength if she is to survive the treacheries of war and grasp her true fate.