It’s not so much that the plot “changed”, but rather that Martin wrote the outline b/c his publisher (or whoever) really, really wanted an outline. Martin said he was “making shit up”.  He seems to have liked the idea of a non-Targ (at least not officially seen as Targs) pair of siblings shagging like bunny rabbits in heat though. And yes, that’s Jaime and Cersei. I mean, Cersei didn’t even exist in that “outline”.  The most interesting thing he did w/ all of this IMO is that we have t