The siren might start as a mysterious and somewhat menacing figure. As she gets involved with the villain's family, she could gradually soften. For example, she may develop empathy through her interactions with family members. She could also learn new values from them and start to question her own motives.
One possible plot twist could be that the siren, initially thought to be a tool for the villain, actually has a hidden agenda to redeem the villain's family. Another could be that the siren discovers she has a long - lost connection to the villain's family that changes the whole dynamic. And perhaps the siren's presence causes unexpected positive changes in the villain's family members, making them question their villainous ways.
The villain with a crush might show conflicting emotions. They could be cruel in their usual actions but soft and vulnerable when it comes to the person they have a crush on.