A:The condition is that the father in law touched his daughter in law with a bad intention. If he only had a bad intention but did not touch her, the Nikah with the son will not break.
If he touched her with a bad intention, the Nikah with the son will break forever. Unfortunately they can never marry again ever. It is not the fault of the daughter in law but the father in law will carry this burden of breaking the home of his son.
Allah Ta’aala knows best.
Moulana Yusuf Laher