DominicCobb said:

The triangle practically disappears from ESB because Luke spends most of his time away from the other two. Yeah, Leia professes her love for Han in ESB, which would seem to put a fork in it, but if she wasn't made Luke's sister, Luke still could've had interest in her which would have made for some interesting material in ROTJ.

They could have done that but it would make things very weird after what we see in ESB. I mean if Luke attempted to make a move on Leia and if she responded to it, then it would establish her as kind of unreliable "whore" character. If she rejected his moves (as one would expect) then it would make Luke look completely lame. I think overall, the "sister" solution was decent in the given circumstance.