Right. Feng Tianyi had taken a risk and came clean to her, didn't he? If he was really planning to deceive her, use her against his half brother, Feng Tianyi would have instead done his best to prolong the revelation of his true identity.
Not only did he admit it, but he also didn't force her to cooperate in seeking revenge on Feng Tianhua. On the contrary, he had encouraged her to do whatever she wanted by helping her secure her position at Tang Enterprise.
Her acquisition of the company was made possible by him, and Tang Moyu knew this. If it wasn't for his help, she was sure that it would take longer for her to secure her position in the company.
However, the fact that he was Feng Tianhua's brother and his attempt to hide himself as Qin Jiran to get close to her and her children made her wary of him. Whatever his reason was, it won't change the fact that he'd hide this information from her.