In Plain Sight by Julie Trettel

Bran is filled with hate and anger over the deaths of his parents and willingly accepts a mission to avenge them. The mission is simple:
1. Go undercover as a rogue shifter looking for work
2. Infiltrate Collier Pack knowing their close ties to Westin Pack
3. When the opportunity arises…Kill Kelsey Westin
His mission is sealed by a blood oath forged by a powerful witch. He’s prepared for every possible scenario—except her.
Ruby has watched as one by one her sisters find their true mates. They’ve fought the calling and she’s seen first-hand what an unresolved bond does to a person. Vowing never to do that to herself, when she stumbles across a handsome stranger squatting on Collier land, she knows he’s her one true mate. And she wastes no time sealing their bond forever.
Now Bran is bound to Ruby, and her position gives him quick access to his target in unexpected ways. Killing Kelsey now comes with a steep price of losing Ruby forever. But the pain of the blood oath will kill him if he doesn’t act.
Time is running out. Kelsey or Bran must die to break the oath and true love cannot fix Bran’s dilemma…or can it?