Sin The law absolutely should be involved in such incestuous cases.
Not sure incestuous is the big issue here, as while enforced chastity concerns a person's sexuality (and therefore falls under sexual consent rules - freely given, revokable and all that stuff), it is not a sexual act unless you make it one.
The far more critical problem is the power imbalance, especially if the wearer is a minor or the parent otherwise still has a duty to care for the wearer as a dependent. In that case, using that relationship to negotiate the terms of a chastity agreement - which would always have to be assumed thanks to the aforementioned power imbalance - can quickly end up qualifying as sexual assault or any other crime in that category.
