Responses
I think it might be more about how to make yul feel secured she(the mother) may have avoided sui intentionally or unintentionally while being attentive to yul. Cuz if it's about affairs a child's grudge is mostly a result of observing the other parent's distress. it's a spouse's resentment or grudge or pain to hold on. Which doesn't seem much likely as of now.
Yul has some audacity to ask her to visit their moms grave. Sui should despise her mom and Yul is nothing but a physical reminder of her mom's affair so I can understand why she would hate him too despite him actually being innocent.