I undertsand how hard it would be because it can happen to anyone with any possible future babysitter: friends, aunties, grandparents etc. I remember offering to baby-sit my friends kids years ago and only realise now how unprepared I would have been - not in the physical sense but with just knowing what to do.

I think when kids are young the person who is looking after them has to have some idea about what they like/dislike, how they operate, their routines etc. They get to know this by spending time with your kids and you and your partner. Once you feel like they have a good understanding then you might feel comfortable leaving them. I don't think it matters how much "stuff" they have (though it can help!) but it's more about their attitude and if they are willing to listen to you and your requests.

Offer for you MIL to spend more time with you and maybe suggest that with your youngest being so young still maybe she needs to get to know him better before he has an overnight stay. Tell her you don't want it to be a stressful situation for her because it would be if she simply just took them and did whatever.

I really don't think it matter who the baby sitter is, as long as you are comfortable then you can be happy.