[SPOILER]

I'm not very fluent in Indonesian, but I can tell what I understand
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To all fujoshis out there, sorry to disappoint you,from the flow of the story up to chapter 60 something, it might be not BL. Yoona has known his brother is not his real 'blood' brother when she was 6, and said she wanted to marry him in future (because she knew they can get married) but their mother opposed it, because she insisted that he was her real son, no matter what. So she asked Yoona to keep it as a secret that she knew they are not real siblings. Then when she grows up, she unconciously falls in love with MongRyong because his actions are very similar to her brother.

There are some misunderstanding between Yoona and MongRyongXSihyeon ;
1. Yoona thought they are a couple and love each other although expressing their love through bickering
2. The reason her brother put an end to his previous engagement and not accepting any love confession and marriage arrangement by their parents because he is gay.
3. His brother is just a plain jerk and really hates to have an embarassing sister,when he asked her to study and get perfect scores. He actually had an agreement with their mother after he broke the engagement (because the younger brother of his ex-fiancé was a bully to Yoona). The agreement is to always keep her sister excel in study or else she will be arranged to be married to some conglomerate's son when she is old enough.
4. Yoona confessed to MongRyong and was rejected because he is still unable to love other people. Yoona took it as MongRyong still can't accept anyone other than her brother. The reality is MongRyong still couldn't forget the noona he killed when he was in orphanage (he did end her life support because she couldn't bear the pain)

Up to latest translated version, MongRyong and Sihyeon are just involved with Sihyeon's family politic. And yes MongRyong has some connection with Sihyeon's mother's family. There's no indication that MongRyong will have some love interest or get involved in some love relationship.

Btw so far there's no gay character in this webtoon. And everyone has his/her past love story told in the process (I'm still waiting for Secretary Kang's story though lol).