<human>: Can you give me a shortened version of the text?

As the title says, we found out that my step-son [8M] was playing the kissing game with his adopted sister [8F].  Apparently, they've done this before a few years ago (I wasn't around when that happened so I can't elaborate on it much) and were told then not to do that.  They both have known each other as siblings since they were 3 years old.

 

Now that they're almost hitting puberty, how do we have a discussion with them that dissuades romantic feelings and gestures with close family members?  What should we tell them, or how should we discuss it in a matter that most effectively turns them off from having romantic feelings for each other and acting on them?
<bot>: My step-son [8M] was playing the kissing game with his adopted sister [8F].  How do we address this so that it doesn't happen again?