Oh OJ - this is not what you need. Is there a professional, eg SW, who could mediate between you and MIL - in a proper conversation at a neutral location - so it's not in front of Steve and you can both put your points across?
At the moment I fear that you are painted black by all the relatives, and they are not going to listen to any of your reasons for the visiting rules in place, as they see the rules as being made just to pick on them.
I hope you get some sleep tonight - tomorrow is another day
Love to you, Steve and the kids
T x