Siobhan's talent seems to be similar to Jasper's, because it is in the ability to alter moods. On p. 660 of Breaking Dawn, we learn that "Carlisle believed that Siobhan had some subtle but powerful gift to make things go her way - and yet Siobhan didn't believe it herself." So apparently, she has the ability to want something and make it happen. In the fight, the rest of the vampires want Siobhan to hope for a peaceful resolution, in order to make that resolution come to fruition. Yet, as it says, Siobhan doesn't believe in her own talent, so it is unsure whether or not she can actually control it. She still tries, though; on p. 677 Bella narrates, "I noticed Siobhan rubbing her temples, her eyes closed in concentration...." Since the ending of the novel works in the favor of the "good guys," maybe Siobhan's talents were useful after all. :)
Carlisle said that Siohban had a gift for making things go her way or willing them to go her way. However Siohban always laughs it off and say that she doesn't believe she has a talent but rather plans well. But Carlisle replies that she has a really good ability to plan well. But I think she does have the ability to make things go her way since in the back of Breaking Dawn where it list all the people who have powers with an asterisk and her name has an asterisk.
carlile believes siobha has a gift though she belives she does not carlile believes that when siobha makes a desicion she can almost ''wiil'' them to be so.Though she does not believe it she humors him and tries to ''will'' it that it will not come to a fight with the volturi-then this leaves you wondering doesnt it? if the volturi didnt attack because siobha ''willed'' it or because alice showed up to the rescue=]