Dump the first sentence. It’s too non-specific. The next sentence is good, but the rest is, again, non-specific and confusing.
When her unknown father’s locket turns up, Tamin Moon is thinking only of the father she never knew. But when she opens the locket she releases an ancient power, bent on renewing an ages old conflict between X and Y.
Armed with the power of the locket, Tamin is on the run from both sides. In a world that shouldn’t exist, Tamin must choose sides, but how can she choose when she can’t trust anyone, not even the man she loves?
Something like that.