Honestly, I get it, OP.
I don't have kids nor do I want them, but I live alone and I like and enjoy my own space, and if my brother had to move in here and my nephew (who I love) had to visit every weekend, I'd feel like pulling my hair out.
There's nothing wrong with wanting to enjoy your space and peace at the weekends. I have real difficulty with people who don't understand this and seem to equate a desire for space with a lack of love or care for your family. That is not the case.
You've already done the job of raising your children. Now, every weekend, your days are dominated by your GD. Not remotely her fault-but does your DS do anything with her? Or is it a case of she gets dropped off/picked up and dumped in front of the TV all weekend? And you're expected to mind her whilst he does his own thing?
I don't think you can make a suggestion for him to see her less, but you can definitely place some boundaries in regarding your space. Like request when she is there that he does things with her-takes her to the park, goes to the cinema or wherever she would like to go to-spends some time with her outside of the home and gives you time to yourself.
What's his room situation like? Where does she sleep when she stays over? At nine, is she allowed her own room/space where she could have a little TV set up and a bit of privacy for herself? I know at nine I really enjoyed my own time and space in my own room. I know that's a lot and it may not be possible/feasible, but if it is, it may help ease the transition whilst he is there.