I don't have anyone in my DS's life who pushes religion on him (apart from one of his school friends) but I came upon a solution to this by accident. We were leafing through a Horrible History book about vikings and we learned that vikings believed that clouds were the brains of dead giants. DS had already learned about clouds and rain (thank you Blaze and the Monster Machines) and was really shocked that the vikings thought something so blatantly wrong. That led to me explaining how people in the past made up stories to explain what they didn't have the scientific knowledge to understand fully.
So ever since when questions about religions or god come up I refer back to giant brain clouds. At this time of year a lot comes up about the cheezest christ baby. So I explain about the axial tilt and how most cultures in the high northern hemisphere developed mid-winter celebrations to help them through winter and stories to explain the phenomenon, Yule, Saturnalia, Dong Zhi, Shab-e Yalda, etc. That Christmas is the name our culture uses due to the dominance of the christian myth of the birth of Jesus, even though there is no actual way that happened in mid-winter, if it happened at all which isn't likely. Then we explore what aspects of our Christmas come from where, Christmas trees being Germanic/eastern European, Santa who first came to vikings in Scandanavia, Yule logs/holly/mistletoe being celtic, festive lights being an almost universal part of the celebration of the passing of the darkest night, etc.
Knowledge really is power when it comes to religion. Knowledge of the real scientific phenomena. Knowledge of as many of the different stories as possible that various cultures have used to explain what they couldn't understand. Knowledge of all of the different ways these stories have been adapted and bastardised to make them more palatable and believable as human understanding grew in ways that would otherwise diminish their relevance. Knowledge of the different ways that religion sought to suppress real knowledge when it contradicted their stories.
As for dealing with your in-laws, I tell my DS that people's beliefs are important to them, so try to respect that. But if someone doesn't respect what he believes, he is free to be frank. Get your DH to talk to his parents, as it should be him dealing with this rather than you. Ask them to respect your family philosophy, explain that if they won't you will have to respond accordingly in ways that you would rather not. If they start telling your dc about Jesus, you can interject and explain that he almost certainly never existed as there is absolutely no primary or secondary evidence of him, no evidence of many of the events that supposedly happened in the new testament, that the various texts of the new testament contradict each other and that there is evidence of the main seven times between 325 and 787 that the bible was re-written to suit the rulers of the church at the time and shape the basics of the various Christian religions we now know. Your children don't need to understand it all, the odds are that once your in-laws are faced with the consequences of how disrespectful they are being, they will stop.