As you probably know, a modification to the parenting plan can be accomplished in one of two ways; by stipulation or by court order.
As to stipulation, this seems unlikely as, based on your information, it seems unlikely the father would consent to anything.
As to court order, this would be obtained by demonstrating a substantial change in circumstances since the entry of the last order. As the order was entered about six months ago, this could be difficult. Having said that, the allegations are quite serious, certainly the court would have serious difficulties with the father being involved in a sexual relationship with a 17 year old, especially with your children being present.
You would need to file a petition for modification with the court. Ultimately, it would be up to the court to determine if there was a significant change and if that change warranted a modification of the parenting plan.
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