On my second playthrough, when I was introduced to Cailan by Duncan, I noticed that Cailan actually bears a very striking resemblance to Alistair in terms of facial features. I can't believe I actually never thought that the 2 of them might be brothers when I was on my first playthrough before Alistair made the confession. Heck, I didn't even take note of the resemblence then. Is it just me, or does anyone else feel the same way?
One of the devs said they used the same face model for both characters. Alistair's face is just an edited model of Cailain's face. Also Cailain and Alistair have different voice actors. Steve Valentine(The voice of Alistair) is voicing Anders the male mage companion from awakening. Anders looks very similar to Alistair and Cailiain as well.