Vader only starts sensing Luke after he gets some training and goes from being merely Force sensitive to a proper Force adept. Trained adepts are consistently shown in the franchise to be easier to sense than untrained sensitives, like Leia.

Vader only knows Luke is his son after being told his name. He only finds out Luke has a sister after ripping the knowledge from his mind, and it's unclear if he got her name.

In 4, Vader senses Obi-Wan's presence on the Death Star, but not Luke's.

i'll admit it's been a while since i've watched the films, but it was probably a bit more obvious with Luke, right? he wasn't exactly hidden. he had vader's last name and lived with vader's family on vader's home planet.

meanwhile leia was adopted by some rando and had her name changed to match. she probably didn't even know she was adopted.

anyway, i will have to rewatch the OT with this in mind at some point.

Vader can sense Leia at the end of 6.

He can? The only thing I remember is him reading Luke's thoughts that she was his sister, which doesn't involve sensing or reading her at all.