How to set boundaries with in laws without coming off bitchy?

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I'm not super close to my side of the family and I have no problem laying down things with them but my in laws are another story. My SO is very close with his family and we live next door to them. My MIL keeps making comments about how she cant wait to see her grandson every single day and wants to have Nana nights where she keeps my son overnight and Nana days where she keeps him all day. I will be a stay at home mom so I'm not good with this at all, Wtf am I supposed to do all day and night without my son and why would I want to be without my son anyway?? I've talked to my SO and he shruggs it off like they are just excited. The real problem is MIL has started adding in her advice, not really advice but "how I need to raise my kids right." She's very religious and I am not at all and she thinks that if I don't raise my kids the "Christian way" then I will be messing them up. My SIL let my MIL raise her kids until she moved out of my MIL house after divorcing my BIL and still drops her kids off 3-4 times a week with my MIL to take care of. I will not allow someone else to raise my kids and my fear is that since my MIL has so much control over her other grandkids that she is just going to expect it with my son. I don't want to be bitchy or come off rude to her but how do I in a sense "lay down the rules" with her before he gets here so I don't have to deal with it when he gets here?