I do not think that you can get a restraining order, but you shoulder with an experienced family law attorney to determine whether the Judge can intervene as part of the custody proceedings.
My response is based solely on the limited information contained in the question. It is not meant to substitute your attorney's advice.
This isn't something the courts will give a restraining order for. In fact, the only real restraining order available in PA is a Protection From Abuse, which applies only in Domestic Violence situations.
What you're talking about is simply a request to a Family Law judge to order a stop to alienating behavior from the other parent. Most states, including Pennsylvania recognize that Parental Alienation is a serious problem and a danger to the well-being of children. California even defines at as Child Abuse. But what you need to know is that it is a disorder that has a remedy in court.
First, I recommend that you research "Parental Alienation" and "Parental Alienation Syndrome" on Google and read up on it. Then, you can described your concerns to the court and how you feel that you ex-husband is interfering with your fiance and his family. What you really need to do is avoid stooping to his level, and thereby becoming an alienating parent yourself.
Using language like "vicious lies" tells me that you are frustrated and upset, but you need to remain calm, no matter what he does, and remain reasonable in your presentation to the Court.
I recommend that you contact a reputable and experienced Family Law Attorney in your area for more assistance in this case and in presenting your request to the Court to stop this behavior and to put it to rest quickly.
Attorney answers to questions are for general purposes only and do not establish an attorney-client relationship. This answer is meant for informational purposes only and should not be construed as legal advice within the bounds of a professional relationship. It is always best to seek counsel with a competent attorney experienced in your area of issue and fully informed about the facts of your case.
In custody hearing, the Judge has wide latitude to hear all issues that may bear on the best interest of the child/children involved. Often is the case where bold allegations are made against third parties with no credible evidence to support same. You have a couple of options, first, your fiance and his family may hire counsel to make sure that your ex does violate any of your fiance's rights. Second, if you are able to show that your ex has no basis to make such allegations and further that your ex is doing this only as a form of harassment, you may be able to have your attorney file a motion for protective order to stop your ex from making any further allegations about your fiance and his family.
This answer in no way constitutes and attorney client relationship and this answer shall not be considered legal advice.
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