Yes. They date. And they fuck as well. I didn't read the entire novel though, just skip skip skip. But, from my "skipping", it seems like the family knows. Idk what their parents think about their relationship but, in my point of view regarding to their sister, seems like she's opposed to it....??? Idk, not sure. Anyway, here's the link (although I only got this from the comments as well, lol):
https://duskblossoms.com/novel/beastly/

Wait... So you're saying that their kids are dating each other???