Outcome manipulation

"Though Siobhan does not believe she has any talent, some suspect she can affect the outcome of a situation through willpower alone."

- Entry on Siobhan's gift

Outcome manipulation is Siobhan's subtle but powerful ability to change an event or course of events by visualizing it with hard concentration. It is known as one of the most powerful abilities known.

History
Though Siobhan does not believe she's particularly gifted, Carlisle Cullen believes she shows this ability and can manipulate the outcome of situations, by wanting them to go a certain way and concentrating hard enough on this desire. Although Siobhan herself does not believe she possesses a gift, Stephenie Meyer marked her name with an asterisk to show she has a "quantifiable supernatural talent" proving she does have an ability to manipulate the outcomes of an event.

During Breaking Dawn, Siobhan wished for the confrontation with the Volturi to not turn out into a fight. As it turned out, she was correct, since the Volturi left without attacking anyone other than Irina.

This gift shows great potential. As in time, and with practice, Siobhan could probably learn to manipulate events with little or no effort.

Another example of Siobhan's power is when Maggie joined her coven. Liam (Siobhan's mate) was very territorial, but Siobhan willed it to work out and it did in the end.