Either you ask them to stay at a hotel, or you establish house rules, then make for some allowances, as a gracious host does. Some of what she is doing is just plain rude, other parts are due to her having a two year old, being out of her own four walls and having an oblivious husband.
If your house rules are to put soiled diapers downstairs, I think its reasonable. But realize, that there might be an occassion where she has just removed a diaper, then has to go chasing after her two year old to keep her from pulling down your china, in which case, you turn a blind eye to the diaper. If she makes a modicum of effort to clean up at day's end, or nap time etc. be pleased for it.
Yesterday, my own mother got after me for not having put DS's stepstool into the corner of her ground floor bath, where it belongs. I said, terribly sorry for imposing on you. I was busy trying to convince DS that he must poo in the toilet, not the sink, all the while trying to avoid a tantrum, and get to your dinner table on time because you wanted us over for dinner and had called company. Next time, I will try to be more mindful of the stepstool.
Its tough work on either side.
Best,
F. B.