Getting the full story, it seems unfair to call the OP rude for providing necessities for herself and her family. Expecting somebody to go without basic necessities such as food, drink, or plates and cups to use or labeling them 'rude' for providing these items for themselves boggles my mind. SIL has seen the consequences for her lack of planning and it doesn't seem to bother her that her guests do without the basics. The OP is making the situation manageable for herself and her family by providing the food and utensils that the host refuses to provide. I don't see how that is rude and making such an effort to spend time with her DH's family seems to show a lot of devotion to her DH on her part.
OP, how often do you and MIL make the same dish for a party? It seems a bit unusual that this would be a coincidence (unless you both tend to make the same dish for all potlucks). I like the idea of calling her ahead of time. If she doesn't know what she will make, then you can tell her what you will make.
I do think the OP should bring containers she won't miss if she wants to avoid a confrontation, but not go out of her way too much. It isn't rude to tell MIL and SIL that they cannot take your pot. If they don't have a container to store leftovers, too bad.
