I'm getting a bug in SOD where Sirene will say something and then the conversation will end once I click "Continue". I had a similar problem with Roxanne's Sandrah companion in SOD, and she gave me a hotfix which fixed the issue.
Pure speculation (probably wrong): This is quite possibly down to the mod which turns the SOD 'overhead' banters into traditional, textbox ones.
Great companion, by the way. Plan to take her with me everywhere regardless of this bug.
Is this BGEE/SoD or EET?
And which is the NPC addressed by Sirene?
Because, Sirene is not covered by the banter conversion mod. The mod author of Sirene has chosen the overhead texts for his mod. It is strange that she tries to open a dialogue with an NPC, if it happens in EET, it may be some *leftover* trigger from BG1 where Sirene banters *normal*.
BGEE/SOD with the EET mod.
I can't be certain, but judging from the tone of voice she adopted it was one of the evil companions. Edwin, Dorne or Viconia. I didn't have Edwin or Dorne with me, so that should remove them from the list (unless the bug is bigger than I realised). It's also possible she was speaking to me since I had low reputation from recruiting Viconia and the goblin, but it didn't seem like it. I wasn't able to read the entire conversation, and wasn't expecting it to close so I didn't pay much attention. Another possibility is that either her BG1 or BG2 banters have leaked into SOD. If I remember right she did have one overhead banter in SOD earlier in this playthrough, but she's only spoken in text boxes since then.