I am writing a fantasy epic and one of the plot points is that the MC has the youngest of his older sisters flirting and noticing they are attracted to each other. The MC one night then invites her to join him for wine after dinner to get to know each other better as they seldom see each other. As the evening goes on, they ultimately sleep together, and for a brief period of time they have a purely physical affair.

What i have written down is that they live in a palace a mile by a mile in size and have rarely even seen each other, never mind interacted till the MC is roughly 18 and she is 20. Especially considering they were raised for different reasons and often were at separate ends of the castle or even the city state they lived in. I know this is called the Westermarck effect and because siblings aren't near each other often enough, they see each other as potential partners. They also have very overbearing parents and are held to high standards to live life on the straight and narrow. Could all of these be enough to make such an event as i described earlier possible, or would there have to be more going on outside of those three factors?

Thanks in advance!