Op, I completely understand. Mil is similar and there is simply no question about length of time or where she will stay. I realised a long time ago that I needed to accept it ans roll with it or my relationship would die.
Having said that, rolling with it does not mean rolling over. Here are the things that have worked for me:
I am hospitable and helpful but I do not adjust my life for her. I cook meals as normal, run life with my usual routine etc.
Dh is responsible for her. I get on quite well with her actually and will happily chat or enjoy a glass of wine or spend a day with her but... I will not be sucked into sorting her out or ensuring she is happy. I am supportive and helpful but only in as much as it is convenient to me. Eg if she is visiting sil and needs collecting and its convenient, I will collect her. But I will not delay children's bedtime, adjust my work schedule etc to suit her.
She is in my house for a long time so I continue to.gice myself.time out as normal. It might be that I disappear to bedroom to watch TV rather than remain in lounge but I 100% feel no need to entertain her.
After her last trip, I have added a line in the sand whereby dh has to be more considerate of the dc. Because she is very demanding of his time and the d were starting to get upset that they could never do anything with dh as 5 minutes in she is asking him for something.
I probably sound heartless but I really am not. But for me to survive having someone else in my home for that long against my will has required very strict boundaries.