An FxF with the Oracle's dream does work out rather nicely, more so if the twin in question that is being tormented by these sudden urges does have the magically abilities of shape shifting.

The idea of their parents being from two different nations could further help the plot, thus giving both twins powers too, and not just having one with the ability to see the future... A war would also be the perfect reasoning to why perhaps these unnatural feelings are rising - perhaps blaming the dark forces at hand. - Also due to the nature of the Oracle she has never been around anyone quite as much would could easily boil her forbidden love towards her sister; to which she is unknowingly already lusting after her.