@Facing40andfrazzled
I'm assuming that more things happened between the dress fiasco and now to make a more logical story as to how you got where you are.
Be that as it may, if I'd gotten to the same place you are, no, I wouldn't let them use the field. To do so is to basically give them tacit 'permission' to treat your family like shit and walk all over you because they know there'd be no real consequences. In fact I'd be binning off the whole lot of them and going NC. You don't need this kind of hassle in your life.
Being as it's your DH's family that does make it harder because your DH has a right to make his own decision as far as what, if any, contact he wants to maintain with them. But I'd be telling him that I had no intention of ever being in their presence nor speaking to any of them again. And that I'd be doing my level best to keep the children away from them too.
At this point all you can do is tell your DH that you, personally, are revoking your permission for them to use the field, and that if he allows them to proceed he is doing it against your express wishes and he will need to bear the consequences of doing so. He needs to decide where his primary loyalty lies, with you and your children or with them.
Also, if in fact they are badmouthing your business to your clients or potential clients I'd be seeing a solicitor.