Situ Haoran was different from Situ Haotian, he had his own concerns. After all, Liu Chengtian was his wife's nephew. As his uncle by marriage, he couldn't just confront his wife's nephew without any evidence.
Situ Haoran's concerns were valid. However, Situ Haoran's attitude made both Situ Haotian and Situ Che feel very uncomfortable. Situ Haotian directly yelled at his brother Situ Haoran, "What more evidence do we need when it's all so clear?"
But Old Master Situ agreed with his eldest son's opinion, that no matter the issue, evidence was necessary.