Did your parents make their plans before MIL? If so, then they get precedence. And is your MIL planning to stay in your house? If that's the case, explain to her that you cannot accommodate both your parents and her at the same time in YOUR house (about which you should have the last word, remind her.) Tell her that if she wants to stay at a hotel, she's welcome to visit, but you will not kick your parents, who have already planned their visit ahead of her, out of YOUR house. Make this about logistics and the facts rather than her lack of consideration.

I guess if your parents have already bought their flight tickets, their plans can't be changed, whereas hers can. Tell her you just don't have the room to accommodate her and them and she'll have to come another time. Be firm, but provide her with some alternative dates, so she doesn't feel like you're fobbing off indefinitely...though that might be tempting given her obstinacy!

You don't mention if there's distance involved here? Do you mean your MIL wants to visit you or she wants you to visit her. If MIL isn't far from you, you could take your parents to visit her or invite MIL for lunch. if you're parents visit was planned a while back & there is distance involved you should just explain to your MIL you'd 'love' to visit another time or have visit u xxx