I really have never understood the problem some have had with Leia and Luke being siblings. …there’s nothing that contradicts it.
“I know. Somehow…I’ve always known.”
Still doesn’t contradict it. I don’t count anything from the comics or anything else outside the films so I don’t care about your first example.
And as for the Empire kiss, she did that solely to make Han jealous. Not because she had a thing for Luke. Clearly, she was in love with Han. If it wasn’t obvious then it definitely was by the end (“I know”) of the film, around the same time her force sensitivity and special connection to Luke is also revealed.
And the “always known” line meant that she always sensed some kind of connection between the two on a subconscious level but was never able to rationally explain it or put it into words. Now that Luke has revealed they are siblings and she understands her connection to the force, it all makes sense to her. That’s how I’ve always interpreted that line, not that she literally always knew they were siblings.