Disclaimer: I don't own Supernatural and never will. No copyright infringement intended.
Amelia's plan is working perfectly and it's gone off without a hitch. Sam Winchester is craving a normal life, something that will tear him away from hunting – and his brother. It was a coincidence when he came in with the dog that day, but she saw an opportunity and ran with it when she guilted him into keeping Riot. Who could resist taking down a Winchester, especially a vulnerable one? The other one was most likely dead, so she preyed on Sam's grief.
He didn't know that she was slowly taking away memories of the things that mattered – such as birthdays spent with Jessica or Dean (and sometimes John). Amelia was going to be patient – Taking Sam away from the hunt wasn't going to be easy, even when he was alone. She did have some feelings for him, though, and that actually aided her plan. And she pleaded ignorance when sometimes "Lucifer" or "Michael", and occasionally, "Adam" came for a visit – Sam's time in Hell was still causing him issues and she delighted in that. It only made this easier.
Amelia didn't even mind when Sam decided to break up with her because she knew it wouldn't last. He'd come crawling back to her eventually, and she happily awaited that day. She had Riot – who was docile around her because she commanded him to be – for company and her job. But not for long. Her plan was about to come to fruition soon enough and she was excited.
Sure, Dean's reappearance complicated things a little – As did his little stowaway, Benny, but she was willing to let the vampire in on her plan, but it also made Amelia's little long con easier. She knew exactly what buttons to push and they were damn well going to be pushed hard. And she wasn't sure what Benny's game was, but she bet that he was playing a similar game on the other Winchester.
Amelia heard through the grapevine that Dean was angry with Sam for dropping out of the hunt for a year and not looking for him (Grief, combined with her plan) and the brothers were having issues. She couldn't stop smiling the night she found out because it was like everything was going in her favor. That hadn't happened in a long time.
The Winchesters were going to be torn apart (for good this time) and twisted every which way until they broke, starting with Sam. He was going to suffer for all of his actions, and Amelia would personally make sure of it.
His perfect little normal life hadn't been so perfect after all, and she was going to rip them both apart with glee. And this time, they wouldn't come back. The Winchesters' luck had finally run out. There was no running from this. Amelia would make sure of it. But for now, she just had to bide her time and watch Sam and Dean splinter and grow further and further apart by the minute. And it wasn't going to take very long, either. Amelia's plan will make everyone else fear her and she is looking forward to a rise in power.
Author's Note: There's something off with Sam's perfect normal life and I'm not really sure what it is. Looking forward to finding out and excited for the rest of the season. Hope you enjoyed!