I cannot stress this enough. Ignore the second couple. To be honest just skip every scene that involves them or their parents. There are so many things wrong with that relationship it would take forever to name them all. For instance, the mom KNEW her son was planning on getting his brother drunk and raping him and she allowed it. Like TF?!?! For the most part, their love story doesn't contribute anything to the main story at all so you won't miss anything major. Also you'll save a lot of time and have a better viewing experience overall. The main couple is cute and the only reason I'm continuing this show is just to see where they'll end up.

Update:
I have since finished the show and just wanted to add a little bit. I do still stand by what I said above. However just because I didn't personally enjoy or feel comfortable watching their scenes doesn't mean you would feel the same. Definitely watch the show and see whether you would prefer to skip their scenes or not.