why do serena and dan want to date each other if they're brother and sister?

They're not brother and sister. dan and serena got together before they knew scott existed, before they even knew their parents had a history. they had really strong feelings for each other before that, and its not like those things happening make how they feel magically disappear.
DS are perfect for each other.

MTE !

Everything and anything can happen in Gossip Girl's world. Come on Serena hooked up with her bestfriend's boy, do drug, killed a guy, and now she can't date who she love ? Who care they share the same parents, have you heard Dan call Lily "mom" or Serena call Rufus "dad". NO, their parents got their happily ever after why can't they.

And another thing, Rufus and Lily being together just proof that that is how Dan and Serena surpose to be in the future. It's faith

No Serena and dan are not related but they share a older brother with kinda makes them related and then there parents lily and rufus get married so they share a brother and they are step siblings and they eventually share chuck bass as a adopted slash stepbrother.