How do I respond to my ex taking me to court for modification to child support and terms of custody?

You don't supply several key pieces of information.  For example, ARE you "harassing his girlfriend" without some legally sufficient cause?  What connection, if any, does that have to any interactions relating to the children? Generally, interactions with third parties are irrelevant to custody and support.  Best thing for you to do -- presuming there is no such connection -- is document the threat, in writing, so if a baseless allegation regarding the kids is made you can show its actual motivation. If, as is likely, neither your question nor this response captures what is really going on, you should consult with a family law specialist.