First of all, it is not 'perfectly normal' in Japan to be in an incetuous relationship. Secondly, it's blatantly, blindingly obvious that Wakka, Lulu, and Yuna are not related.

First of all, Yuna is half Al Bhed and half Spiran, right? So to be related to her, Wakka (who hates the Al Bhed) would have to be part Al Bhed. He also looks nothing like either Lulu or Yuna, his family is far darker skinned than theirs. Yuna was born in Bevelle, and then taken to Besaid after her father had defeated Sin, where she met Lulu and Wakka. They say many times that she is 'like a sister', and you wouldnt refer to your own family as something 'like family'.

"You were like my brother and sister... no, you are my brother and sister." it's just a way of saying how much she loves them and that, even though they arent blood related, they are still family.