I think it could be as she said. She saw how the true wife treated her children and realized she loves her children more than her position. Plus it probably helps that now the heir is out, there's less stress upon her children and her position. She's just happy that her children are happy and in good standing than risking it.
well from the 3 people one never hurt the mc and helped him(and the mc manipulated him) another was show to be bad but never did anything to the mc(until this chapter) and another has taked advantage from the mc every chance she gets without feeling any tipe of guilt from it so i think is obvious which one deserves to die more than the others because atleast the other 2 are redeemeable but the one who always takes advantage of him? no she deserves to die