You should be blunt with her, and say (for example) that you just don't have the time to make it weekly, and you'd be happy to schedule it for once a month.
Don't argue back, or feel you have to justify yourself if she gets angry.
I'm wtih CAT on this....you need to let her know that while you love her, this is getting a bit too inconvenient for you & your SO.
I agree with Cat and Helga...sometimes you just have to say no and accept that the other person won't like it. Of course you should be as kind as possible when you do it and minimize the chance of hurting her feelings. But in the end you can't control if she doesn't like it.
I like Helga's suggestions. One more thing is to try to spend time with her at other times of the week rather than to just cut out one occasion of seeing her. You'll want to balance the time that you spend with her. So try suggestion you and she or all of you get together to go out and do something.
If it were me, I would try to fix the situation - but then I'm a fixer.