The thing about Ruby was that she saw more than anyone gave her credit for. She was sneaky and conniving, but she was also fairly observant. She was so good at what she did because she knew so much more than anyone assumed she did. Ruby had learned how to take what she knew and turn it around in her favor. It's why she worked so well with the Winchesters, because they always assumed they had a leg up on the enemy. And they did, except when it came to Ruby. Everyone else let their egos get in the way, but Ruby wasn't above sinking to their level to get something done. It was easy to do with Sam, easy to play to his daddy issues and his need to feel like the hero. Dean was harder to sort out.
Not impossible, though.
After seeing Dean in action down in Hell, she knew that him returning to the surface was a ticking time bomb. But she kept to her role. The two of them argued and fought. And she kept leading Sam down the path to free Lucifer while Dean kept pretending he didn't like it and didn't know where it was really going to lead. The only thing that remained real between them was how much they all wanted Lucifer dead... and how much Ruby enjoyed Dean getting pissed off that she was sleeping with Sam. That was fucking glorious.
But Sam wasn't stupid either. Or rather, he knew Dean well enough to know something was up. And the longer they tried to lead Sam astray, the more likely it was going to blow up in their faces. Ruby could orchestrate the whole thing to make sure Sam knew the truth... or they could work together to find a way to lead Sam to the darkside. Either that, or they were both fucked and it was probably better to cut and run before Sam turned her knife on both of them.
There was definitely no time to waste anymore. Ruby waited until the brothers were separated and tracked Dean down. "He's not as stupid as you think he is."