TI see that people are getting the wrong idea; it's not like she's the 4th knight of incest and he's fighting to save the world of incest-themed manga.
But it's true that they were both flirting before discovering they were siblings; they're both trying to treat each other naturally, as they would any other close relative, but hormones distort everything. When she kisses him on the cheek, something she might have done with other relatives, it ends up having a different effect, since she was already interested in him and he was interested in her too.
This creates a conflict where both want to treat each other naturally, but the tension prevents it, so they both try not to show the situation, because they are supposedly siblings, It would look bad if they got involved, and if they tried to avoid each other, family life would become a mess, a real dead end. Now all that's left is to figure out which of the two will start praying to God first that they aren't siblings.