Serena's suicide attempt left her emotionally devastated, and she refused to speak. However, the police station wasn't a place where she could simply keep her mouth shut as she wished.
When I heard from Caelan that Serena was my half-sister, I was completely floored.
The reason she hated me so much was that she believed I had stolen everything that should have been hers, so she wanted to exact revenge on me in a crazed manner.
Damien was hired by her. She deceived him, saying that if he killed me in a car accident, she could inherit my estate as my half-sister and would give him a large sum of money. But they didn't expect Garrison to take the hit for me.
The car accident bound them together, so they had to come up with another plan. That was for Damien to get close to me, gain my trust, marry me successfully, and then plot to kill me for my money.