I'm pretty late, but holyyyyy you're in for a ride. The Korean webtoon isn't done yet and I'm not sure how far you've read, but first of all Da-ae and Haneol get together as a couple. They keep trying to prevent Da-ae's sister and Haneol's brother from getting together, but her sister joins as an intern in Haneol's brother's office and he eventually gets feelings for her. However, Da-ae's sister is already dating someone from her college named Kang Sejun (HE'S IMPORTANT REMEMBER HIM) and she rejects Haneol's brother (the thing is, she kind of likes him too....idk this girl needs to get her shit together). At this point, Da-ae and Haneol realize that they can't keep their brother and sister apart and try to figure out another alternative way to prevent them from dying. From the diary, they read that Da-ae's sister was suffering from stalking and decide that they may be the ultimate reason her and Haneol's brother dies in their alternative lifetime. Their main suspect is a shady dude named Gujong, but Da-ae figures out that he's pretty useless and too dumb to stalk someone so they give up on him. At this time, Haneol's brother is in a car accident with a red car (IMPORTANT). The thing is, Da-ae's sister's boyfriend, Kang Sejun owns a red car. Not only that, in their past alternative lifetime, Da-ae's sister and Haneol's brother died from a car accident with a red car. Da-ae and Haneol start to put the pieces together and are like "oh shit Kang Sejun was the ultimate villain". At this point, Da-ae's sister isn't completely dumb as well and had already figured out that her boyfriend was her stalker at this point. She gets into a car with him (I dunno why, she baffles me) and finds all the evidence of stalking in the car (photos, her lost keychain, etc). Da-ae and Haneol realize that Da-ae's sister is in serious danger and go with Haneol's brother to find her. Da-ae gets a call from her sister, but instead of her sister picking up, it's Kang Sejun. He tells her that he knocked her sister out and is driving to an unknown place and that his only request was that Haneol's brother come along too. MAJOR PLOT TWIST HERE: Kang Sejun says something along the lines of, "Taegang (Haneol's brother) already had your sister once, why can't I have her in this life?". Da-ae and Haneol realize that they weren't the only time travelers and that Kang Sejun was the reason why their brother and sister died in the past life.
This is where the webtoon is caught up to, I hope you enjoyed reading my summary (more like a rant lmao). I'll update the most recent episodes if you want me to!