If you had a 'brotherly' friendship with a woman would you give her a long lingering snog to get someone else jealous? It would be cruel if the other person wasn't in on the ruse and Luke clearly isn't.
The scene only works if A) we are ignorant of their sibling relationship and B) so are they.
The line in ROTJ is so awful it's almost like something Frink would add for a very amusing joke. Problems like that need to be fixed at the point they were made. Clearly the line in ROTJ is the problem not the scene in ESB.
You can just imagine Luke hearing Leia say that and then feeling kind of weird remembering that kiss.
The little kisses in ANH were one thing but that smooch...blarrrrgh!