I didn't know where to put this since it's sort of about money but mostly about how to handle something with a relative. Let me know if it should be moved
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So, my mother lives with me and my husband. She is disabled and was struggling financially with the benefit cuts, so we got a two bedroom house and invited her to live with us. It's normal in my husband's culture to live with your mother or MIL and we're all happy with the arrangement. 
I know that she feels guilty about not contributing financially, not that me OR my husband care, so she makes herself feel better by acting as a sort of housekeeper; she does the cleaning and shopping while we're at work and asks us for any little errands she can do.
The issue is that my credit card bill has been much higher than usual, and I've just gone through it and realised that my mum is spending about £170 a week on shopping. My husband and I used to do one online shop a week and spend £60-80. Mum bought two cats with her so I expected it to go up, but not to more than double. The issue is that, because she gets bored in the house all day and because she wants to feel useful, she goes shopping nearly every day. It's just things for the house, food and cleaning supplies and things, it's not like she's buying herself diamond shoes. It's just because, as I'm sure you all know, when you do lots of little shops instead of one big one you end up spending more because you chuck in a few impulse buys and a few treats each time.
I don't know how to bring this up with her without making her feel guilty and without taking away her "job" which makes her feel useful. I want us to start doing one online shop a week and avoiding top-ups unless it's absolutely necessary, which is what me and my husband used to do. We have done this a few times since she moved in and spent about £70-90. But if I do that she will hardly ever get out of the house AND she will start feeling more guilty about not contributing.
Another option is that we agree a budget, maybe £100, and ask her to stick to that. But then I feel like I'm treating her as an employee or someone who needs to be controlled, instead of an equal!
Can anyone think of something I can suggest she does instead of shopping? It needs to get her out of the house (she has a free bus pass because of her disability so can travel around), make her feel like she's contributing to the household (not a hobby club or anything because that would be "just" for her and not contributing to the house) and not be too strenuous?
Or maybe I should just leave it.
The money isn't causing us any problems, it's just reducing the amount we can save.
Any suggestions are appreciated.