If he were your brother, you would have stood up for him when he needed you to the most. But you didn't. And he's had to suffer for your mistake.

[Quietly, coldly, her eyes fixated on Lucifer, unblinking.]

I could care less about your opinion of me, but don't think for a minute that Cas feels the same way you do about what I think.

[Because for all of his bitching and arguing and stubbornness, Rei knows he listens -- and more than that, he tells her most everything. So if Lucifer decides to do this, and Castiel feels the need to go with him, then Rei will go, regardless of what the Devil has to say about it. Because she would rather fight Raphael to the death than have Castiel go through what he did again.]