It doesn't even require prohibiting independent app stores, all it requires is that adding one prompt for a separate password which you don't give to your MIL.
How do you install independent app stores to begin with? Do you install them with he App Store? That would put them at a disadvantage similar to how Microsoft put other Browsers at a disadvantage with IE. Do you provide a choice then when you setup the phone?
Purely from an anti-trust perspective I can see why the idea of independent app stores might seem appealing at first glance. Thinking about this just for a moment brings up so many questions though and I don't see how you can possibly come up with satisfying answers that lead to good security and a good user experience.
Then don't use that one. You don't have to use any but Apple's if you don't want to.
> How do you install independent app stores to begin with? Do you install them with he App Store? That would put them at a disadvantage similar to how Microsoft put other Browsers at a disadvantage with IE.
As opposed to the status quo?