Jih Minghan never guarded against Yin Zhan, perhaps because he could never imagine a reason why his sworn brother would want to kill his wife. He had shared the clues he had uncovered with Yin Zhan, who gradually realized the crisis at hand. Although it didn't currently implicate him and Lin Roujia, as time passed, their connection with the matter would inevitably emerge.
As for Yin Zhan, his own safety was one thing, but Lin Roujia could not die, for Lin Roujia was pregnant.