Just like I told my step brother who is trying his best to shun his Dad, i wont stand by and watch you hurt anyone in this family. I will not condone such bad behavior, nor will i encourage it by not saying anything. Every week I phone him and ask if he has phoned Dad lately.
This weekend he is coming to visit Dad for the first time in 8 months. Be straight with them, get in there first before a pattern of shunning can develop. Make them aware you are watching their behavior and will not put up with it.
If you use their doctrine, they will simply find another doctrine to use against you.
Be human about it! Confront them on a one to one. No wiggle room.
GA