Actually, Situ Ya felt guilty about Duanmu Sheng.
If she had thought to hint to Duanmu Sheng at that time, he wouldn't have been deceived by Shen Haoguang so miserably, nor would he have gotten pregnant.
At the time, Situ Ya was eager for revenge, so she didn't expose Shen Haoguang's true face in front of Duanmu Sheng.
Actually, Situ Ya hesitated for a long time before she ultimately chose to keep the truth from Duanmu Sheng.
First, because any smart girl would have spotted Shen Haoguang's flaws.
Second, even if it wasn't her, Shen Haoguang would have conspired with others to deceive Duanmu Sheng.
Third, to bring down the Shen family, Situ Ya had lain in wait by his side for seven whole years; she couldn't possibly give up her plan for the sake of a Duanmu Sheng.
Out of guilt, Situ Ya chose to save Duanmu Sheng.
But Situ Ya never imagined that Duanmu Sheng would actually carry Shen Haoguang's child.