Filing for custody is not going behind anyone's back. It must be done by filing and serving a lawsuit for custody on the other parent---so it won't happen behind your back.
Technically, you as the parent may take your child from the mother in law if you want. Do realize that it will start a war---which is why I suggested that you go through he legal system.
The reality of child custody
when parents don't yet have a custody order is that for the most part, possession is 9/10ths of the law
. In other words, you have your older son and you can deny your husband visitation any time you want. His sole remedy is to go to court. The monster-in-law and your husband could do the same thing to you as regards XXXXX XXXXX son, and leave you with nothing until you then went to court for relief. The police will NOT involve themselves with this civil dispute.
So, be sure that you focus on getting the court to set out your rights---then if something goes bad---you merely would file a Motion for Contempt with the court and the court would enforce your rights for you---tossing the other party in jail if necessary.
I hope that you found my answer informative, that you are accepting of my efforts and that you will rate my efforts based on the knowledge I have provided to you.
I wish you the best in 2012.