leo87 said:

It's like he just gives it 2 seconds thought and laughs it off. Wouldn't he be like "whoa whoa, how the hell did that happen?" and "You made out with your brother!" It's always disappointed me, but I know it happened at the end of the movie and there wasn't enough time to go into that.

He didn't just laugh it off, he was all like whaaa

And it is super very obvious that the kiss was only to make Han jealous. It was a show. It was not an intimate and sincere kiss. I'm sure later they would have a good laugh about it, but even in Empire nobody took that kiss seriously.

There could have been an extra scene where Leia explains her family tree to Han, but not really necessary.

Also, I love Carrie Fisher. Her performance as Princess Leia in Empire is great. But, every time I watch ROTJ, I feel like she didn't deliver in the scene where Luke reveals they are twins. It's an important scene for the franchise and I think her performance in that scene is disappointing and not what was needed.

I agree that scene was on the weak side.