Install a capacitor, say 100uF, on the output of the bridge rectifier to smooth the pulsating DC out.
Note:
The transfer ratio of the 4N25 can be as low as 20% which means you may only have 6ma for your relay. How much current does the relay coil require? There is really no need for an opto-isolator and bridge here since you have commoned the relay's ground source with the siren's anyway. A diode, cap, resistors, and a NPN transistor should be all you need to drive the relay.