Manipulate the outcome of events
"Though Siobhan does not believe she has any talent, some suspect she can affect the outcome of a situation through will power alone."
- ―Entry on Siobhan's gift
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.
- "Dakota can affect the course of events, controlling when she will next see those she desires to meet"
- ―Entry on Dakota's gift
Dakota can influence when she will meet someone, and that the impact on events this causes will bring that person to her.
Saemus has a similar ability to delay and alter the course of events, Siobhan's gift is a close to but more powerful power, she can change what happens, Saemus can only delay and alter the final outcome