Is a wife who sought a divorce and fled with her children to another country, after discovering that her husband was a womanizer and addicted to pornography, and had harassed her sister, and then her children saw him watching pornography and read his romantic messages, considered sinful?

Watching pornography and a married man having emotional relationships with women is a great tribulation and a greater sin, as it substitutes what is lawful with what is unlawful. You have done well by advising your husband and being patient with him for the sake of the children.

You are not sinful for requesting a divorce due to your husband's transgression. As for your fleeing with the children, this needs clarification: If it was after the divorce and there is a dispute among jurists regarding custody, your sin depends on the preponderant opinion concerning custody (whether it belongs to the father or the mother). However, if your fleeing and travel occurred before the divorce, then you are sinful for that, because you traveled without your husband's permission, and because custody is a right of both parents as long as the marital relationship exists.