Luke had a deep psychological connection with his father. He never knew him, but idolized him. I'm sure growing up he missed his mother, but Aunt Beru seemed to fill in that role nicely. But the idea of his father...oh, that really got Luke's blood pumping. Space travel, adventure, his father seemed to know what was cool. Then to find out that his father fought in the war and was a Jedi Knight! Woo!!

Leia on the other hand, being a female, would have had a yearning to know her mother. What she was like, who she was. She obviously knew her mother because she was with her, even for a short time (sans PT). She never knew her father and to her it didn't seem all that crutial. But her mother, that was the key. Probably the driving force in her life, and she didn't even know it.

Vader couldn't pick up on Leia's vibe on the Death Star because 1) she had the connection with her mother, and 2) she had not yet "activated" her force senses.
Vader did notice Luke, "the force is strong with this one," because Obi-Wan had started teaching him and Luke was now in the Jedi pool and sending out ripples in the force that Vader pick up on. Both Luke and Leia can "swim" but only Luke jumped in the pool at the time.

Luke never knew is mother. Leia never knew her father. Put the two twins together and now they both can learn about their parents.

My line of thinking pushes aside all PT logic (oxymoron). To me, Leia was born first, Luke second. Luke was taken away quickly by Obi-Wan to run to Tatooine. Leia stayed behind with her mother until her death (I assumed a year later when she was murdered or something).

Even if Lucas wanted to have her die in the same movie that the children were born in, Leia should have been born first and in her mother's arms when she died. Luke should have been left in the doctor's arms,having never even gotten to see or even be held by his mother.
Hell, even if Padme had reached out and brushed the back of her hand against Leia's face and have the baby look at her while Luke was held at her feet crying, would have been enough to believe it. But like the idiot Lucas is, nothing makes any goddamn sense.

PS - I thought that this was about actual age numbers of the twins and I was going to say that during ANH both Luke and Leia are 19 years old.
1) Leia was said to be the youngest person to join the senate
2) I saw a poster that mentioned that Luke was the "20 year old son of Jedi Knight Anakin Skywalker"
3) I was also told somewhere, a magazine perhaps, that Luke was 18. I average the two ages together for a happy medium
4) Luke's been denied the Acadamy, again, making him atleast 19.