You have a complicated situation given that the father is a witness. My family that are witnesses know that they are on a very short leash with the witness crapolashine. They have been told they are not to preach to my kids and my kids have been taught to recognize it and rat it out instantly. I have taught them what their misinformed relatives believe and taught them why it is nonsesnse.
It doesn't seem as though you are in the same situation. Limit as much as possible.....no really. I prepared my kids from the get go to deal with these issues and still keep a tight limit on things especially with their grandmother. They get to sleep over once or twice a year for like a night or two. Meetings are absolutely banned. One mention of any sort of preaching that comes across through my conversations with the kids and I grill my Mom like its the inquisition. I let it be known that I was raised to shun and that I would hate myself for ever turning that button on but I would to protect my kids. No one else in my family seems very daring as I am sure that my Mom has shared this info. It really is this important.
I have been out for a long time so this comes easier to me than for most. It took some time to get to this point with myself and to find a way to keep relatives and yet let them know I won't put up with them crossing the line. I would allow contact and then nip it off completely so tha they knew I was damn serious. Had to do it a few times. Never know when I will have to jerk the leash again.
Unfortunately it seems as though you may only be able to limit so much because of the dad.
Hang in there.