Incest in Westeros

So all these talk about incest, that Jon can't marry Dany or Sansa... It seems to me that only marriage among siblings are forbidden, outside the Targaryens, because nobody seems to care about the fact that Joanna Lannister 's father was Tywin's father's brother... Do you think anyone in Westeros, not here, would care if Dany and Jon got married, or, Jon and Sansa/Arya? If Jon doesn't die, of course.