What your mother is doing is not on. How dare she try and use your in laws to get at you. Talk to your in laws. YOu don't have to disclose everything to them but tell them that your mother is not a well women and is trying to cause trouble and that they should block her. If your mother keeps it up I would be contacting the police and looking into a harrassment order been put on your mum. I think you will need proof though, have you kept any SMS she has sent and keep your's and DH phone log of incoming calls so it shows how many times she calls.

Your mother needs to know this is not on and that you need to be left alone. I know at the moment you are concentrating on Sprouts arrival (yay, can not wait) but I don't think your mother is ever going to let up.