I hate the thought of a baby boy being mutilated for any reason, but in this case I would gently advise to let the husband, as foolhearted as he is, to lead his family. Keep praying on it and go to him directly. Perhaps the Lord will soften his heart and give him ears to hear wisdom. But having the wife in the middle will only cause tension between her and her headship. I know this is hard, you have really good intentions and it's frustrating with dispensationalist family members.