Third Person's POV
Seokmin found everything out.
Mara and Shaira share the same parents, they are sisters. He could not make the head or tail of it and accept it, but it is the truth. This put him in an unpleasant circumstance, where difficult choices have to be made. He is thinking about whether he should tell the others about it or not because they do not know the situation yet. On the other hand, he feels that it would be better not to stick his nose on what is meant to happen.
So, he made up his mind to stay quiet, he does not want anyone to suffer pain because of it.
He is highly strung of course when Jihyo opposes on coming with him because the obstinately disobedient maknae wants to stay with Mara.
After breaking the news to Jihyo that Mara left to meet someone, Jihyo immediate
