I call. For Siren to bet the river one of 2 things must be going on:
1. she thinks you dont have an ace and were training something like kj or a mid pp and therefore if siren bets the river she thinks that you will think she has 2pair or an ace and fold your weaker hand. - if this is true you should reraise.
2. She called down a draw that hit and you are no behind. SO, she bets because she thinks you have an ace and she also thinks that you are thinking she donky bet you on the river because she thinks ur hand is weak and the bet will imply she has an ace and therefore you fold. So, because she knows that you think this way she is actualy betting out her made draw (be it straight or flush) beause she knows you will raise and this way she can c/r. - in this situation you should really fold but thats not logical therefore you will rather call then reraise.

So, being that you said Siren is a regular, i would call here and make a note on how she played this hand. I personally will be a lot more pissed off if i am 3 bet and beat by a flush or straight then if i simply call, win the hand, and then think 'i should have reraised!'

I hope this was not terribly annoying to read or hard to follow.