The story starts at the end of episode 3x18 of The Vampire Diaries. Hope you enjoy!
She heard their voices in the Mikealson mansion. She had been following her brother, as always, when he walked inside to get Damon back. Good, she thought, at least they're looking out for each other. Just as Stefan was about to dagger Klaus, she pushed Stefan off of him. Surprised, Rebekah, Klaus, Stefan, and Damon all looked to see who this person was. As soon as they looked at her, all of them froze in shock. "Sydney", Stefan whispered. "Hello, brother." Sydney said. Then, she looked at Klaus and said, "Now, we can either do this the easy way or the hard way, your choice Niklaus." "I'll let your brother go when I get the last stake." Klaus explained.
Once he said that, Sydney's lips lifted into a devious smirk, then she pushed him up against the wall. Klaus squirmed under her vicious grip, but he was no match for her alarming strength as she threw him across the room and into the wall. Soon, she was behind Rebekah with one of the white oak stakes, ready to plunge it into her back. "So what's it gonna be Klaus? Give us Damon, or I kill Rebekah." Sydney said, while Rebekah, whimpering, too couldn't get out of her hold. "Take your family and leave." Rebekah said, trying to keep her voice from shaking too bad. Sydney let go of Rebekah, who ran right next to her brother in fear Sydney was going to strike again.
Turning their attention to Damon, Stefan and Sydney freed Damon, but he was too weak to walk. "Here," she said pulling out a blood bag from her purse, "Take this and go home, I'll meet you there." And with that, Stefan and Damon left her at the mansion to talk to Klaus and Rebekah. Walking into the foyer where they were, Sydney finally took in what she just did. She came out of hiding, saved her brothers, and saved Klaus. Niklaus, the man she loved in the 20's.