I almost posted a complete explanation spoiler here but just stopped myself on time. hehehe. Ran/Lan/Lachia has really great proactive risk management plans.
Soo truee and again lol, isn't this called "slow burn"?
Any other slow burn manhwa/novel focuses on the world building, plot, and other events beside their romance life. From the beginning the plot was thick and always emphasizing lachia, and how important lachiq is to the 2 leads. I don't see where the problem is.
The weirder part is to anyone who started this manhwa and actually hope that yustaf isn't the ML. Like sirrr, its pretty obvious from start so if you continue reading this what do you expect.... if you even felt strange when a "not-too-long-ago-a-stranger-but-lately-become-a-siblings-in-status" happens from the beginning when become "not-my-sister-anymore-so-i'm-gonna-propose" is bound to happen is... errrr
If I remember correctly, from the beginning Yustaf was against calling Ran or acknowledging her as his sister. Also for the lack of chemistry, Yus is unlike other ML who really seduced/flirt with the FL, the story is more focused on Lachia than their romance.