One of Siren's powers is to call for other Warpies to come and find her, which many have done subconsciously. In fact she was discovered (along with her possible brother Snap) by Uncle Lex and they became members of the "Parasites". 
All of the Parasites were trained by Uncle Lex to use their powers to become petty thieves and to obey him blindly, which the children did without question. Uncle Lex didn't value the children under his care, due to his using her passive power,he was able to gather a group of Warpies, and didn't care of any loss, stating there was plenty of them outside, slowly coming to him.
Another group of Warpies known as the Cherubim also heard her call and traveled to London were they met her fellow Parasites, Peeping Tom and Spasm, who brought them to their home, the Den. There the Cherubim found a new home and started to settle in.
Later, when a jewelry heist went bad, Snap became scared and teleported blindly and was struck by a city bus, killing him. Snap's death upset the Cherubim, especially Fern, who were all still mourning the recent death of Cherub Whirlwind.  When they returned to the Den, Fern confronted Uncle Lex and the Cherubim decided to leave them and find a new home. However, Uncle Lex had other ideas and wouldn't let them and set the Parasites against the Cherubim.
Powers and Abilities
Siren is unable to move or take care of herself since she is a baby who sleeps the majority of the time.