Because Vader managed to tell Luke was his son at like 300 meters distance through steel walls and shit, but didn't catch on when he was stood next to his daughter . . . some father!

If Luke had taken on the place of Bail Organa's kid, the rebellion would have been found out very, very fast. I'm sure Bail Organa ran into Vader many times while he was under Empire suspicion, however I believe it's very had to explain if Vader walks into your house and says "What the hell, that's my fucking son! Why do you have my child?!"

Apparently fathers have no connection whatsoever to their daughters. They've got to strangle their son multiple times before they notice they even have a daughter . . . sounds like an episode of the Simpsons.