One of the first episodes of S2 (I believe E1) has Ren getting carsick on a drive with his family. It’s during a flashback when we’re shown how he was as a little kid in Canada. It’s not very graphic though. This happens again in S2 E4. There is a lot of warning, and again isn’t graphic
I would disagree with the other comment (it doesn’t matter if the brothers are adopted or blood related, Haru raises Ren like an older brother would). If incest bothers you I would avoid this show