As far as I can see, there is a lot of unused dialogue in sil.dlg. Several warnings, and also some kind of peace treaty between the sirenes and the PC. In the unmodded game, though, she gets hostile after the first meeting, so triggering a second dialogue if she is still neutral 10 s later will never happen there.
Plus, the sil.bcs introduces a trigger with Shout(1) if shouted by a sirine, only problem is that no sirene in the vanilla game uses Shout(1) (they all do 99). As far as I can see, UB does not change this.
So, as it is, assigning the sil.bcs is rather useless, and leads to sirine's impatient behaviour if combined with LoSC (the only case where the second talking to actually gets triggered).
Since I don't feel like activating any of the unused dialogue (although that would be a nice expantion, making her far more humane than she appers currently), I think the sil.bcs could be taken out without a loss.
(Link to thread about unused script patching, so I don't have to search for it again:
http://forums.pocketplane.net/index.php/topic,20132.msg281333/topicseen.html#msg281333 )