I would just relax and not do anything. I know your upset, any mother would be, but getting a rise out of you is what the person who called in the report intended to do. If the accusations aren't true, which i'm sure they aren't, then you really have nothing to worry about. I'm a soical worker, and I do see these types of cases all the time. People are generally nosy and think they know better than the parent does on how to raise their child, so they think that calling soical services is the way to go about things. Not that things shouldn't be looked into, i'm not saying that at all. There are times when there are things going on, but this doesn't sound like one of them. I have in the past had reports that were totally bogus and everything that was reported was not true, it does happen more than one would think. I wouldn't accuse your mother-in-law unless you can prove she did it, which social services can't tell you because everything is confidential. I would just kind of "mention" to her that "someone" called a report into social services and see how she reacts. If you can prove she did it then you can certainly take action if want. I know both sides to this situation. I'm a social worker, so I have dealt with parents who were totally innocent, so I understand why you are distressed, any good parent naturally is. I am also a mother of a 16 month little girl, so I can put myself in your shoes and understand why you are upset. If your truly feel that your mother in law is the one who made the accusations against you, then you can decide whether or not you want your son around her. As far as legal actions go, you would have to prove that she is the one who made the call with false allegations before you could take action, but if you can then I don''t blame you, you don't want someone in your life that is going to cause more harm than good.
Also, its best to cooperate with the social woker, rather than not, because they can bring the authorites into the case if you resist letting them talk to you or your child. I just wanted to mention that because the poster below stated to not speak with them. Why resist if you have nothing to hide, be cooperative and the process will be over and done with. If nothing is substantiated then there isn't even a case. There has to be proof of some type of neglect/abuse in the home before it can even be cosidered an open case, until then the report is under investigatin until something is substantiated or not. You do have the right to get a lawyer, but they still have to investigate no matter what. I know your mad, I would be too, but its best to just cooperate. They are doing their job, after all the caller is the one who is at fault here, not the social worker.