Little else does make sense, I imagine. It would be pointless to make Cersei and Jaime secret Targaryens, whereas Tyrion hangs around Meereen and has to tame two dragons in the near future. Also, it would make no sense to introduce Joanna as Aerys's true love, or the woman he desired above all others, if this never ever played out. Dany has an affair with Daario, so Aerys most likely shared his blankets with Joanna Lannister once or twice. And later rather than sooner, as the young Aerys and the young Tywin were actually friends [it was rather interesting that Kevan and Tywin still hoped that Tywin would be recalled to court after Connington's failure, by the way], later on, Aerys did no longer like him that much. And we only know that Tywin was all that devoted to Joanna, not if she was that devoted to him. Especially the older Tywin would not have been what you would call the ideal husband.
It might be even somewhat suspicious or interesting that Joanna had no children for years and years after Jaime and Cersei. And in the end, there are Tyrion's dragon dreams, his mismatched eyes resembling those of Shiera Seastar, his platinum blond hair, and Tywin's suspicion that he was not his son.
Judging Tywin, he would never have murdered a child of Joanna's, but he might well have suspected something. But he would have told nothing about that, just as he never spoke about the incest...
Edited by Lord Varys, 12 July 2011 - 07:23 PM.