I'm not sure this will do much help considering your husband has sort of in a round about way ask you to ignore it but he needs to ask her back off. I've been married for almost 8 years and I still hear her bashing me to relatives, I've found nasty emails about me, I've stumbled accross stories where my husband left out the part where he had to tell her to stop talking bad about me...and now we own a family business together with his in laws. She's with my husband day in and day out and talks bad about me often enough that he tells me...he does stick up for me ALL THE TIME. That tells me that it's more than every once in a while.
My advice is work this out now before you get married. Sit down and tell your husband...this has to be addressed because it is hurting you. He needs to ask her to stop or it never will. If you do, she may twist it around to make you look bad. Get outside counsel and keep your mouth shut about talking negatively about her or your husband will feel forced to defend her. Avoid contact with her without your husband or another near by. Miss the phone calls. Don't get caught up in the traps she will set for you.
You may need to fight fire with fire. I'm a Christian and I know all too well some Christians can get a little caught up forcing issues down others throats. She may mean well but this is being done in bad taste. Fire with fire. Go to a church Pastor, at her church and explain that you are unsure how to ask her to tone down her spiritual....ummmmm, tastebuds around the children and you. Tell them you just want a peaceful relationship with her and not a constantly being judged relationship. It's disturbing you and the sleep...etc. Be sweet and take your husband with you to the meeting. Then, set up a meeting with that Pastor and her and have them mediate. You said you have your faith...be faithful to that and again...tell your husband it is his job to ask her to back off.
It's unfortunate that mother in laws get weird over their boys and in this case...go all evil spirits and stuff for extra power...but just do your best to stay right. My husband just gave me permission to let his mom have it if she does it again and I am in the right to say something. I know if I do, she'll say she was kidding and I'll look bad so I'm not sure what to do but having his support makes the different.
That's a tough one. Encouraging toward a Christian life (which is what I can see she is meaning to do) should be like a sprinkle of salt on your food. She's taken off the top of the salt shaker and is destroying the dish so that you can't even swallow a bite. If the moment presents itself, and you can say it in a non angry way, tell her that analogy. She'll know what it means. Ask her to take it easy on you and that she's overwhelming you. Tell her she is coming on so strong that you are wanting to not accept anything she says and you very much want to be able to talk openly to her about everything....but then don't talk to her about it. The lying is a the dangerous part. I'm dealing with that now.
Again, just stay pure in heart and motive. Don't let it consume you. If she does something in front of the kids you have every right to address it....have the kids leave the room and tell her you and your husband do not want her to speak that way in front of them. Call you husband while she is there, tell him you are there with his mother and you just asked her to not do that in front of the kids and tell him to explain that he feels the same way, hand her the phone and stand there. (Be sure he will back you up or it will make you look bad...keep the phone on speaker too)
But before all that, be sure he speaks to her first, alone.
Good luck and keep us updated.