Hi Ribena - I think YABU to post on mumsnet and expect advice appropriate to your situation. No one knows the details and most will be filling in the gaps from their own experience or imagination.
tbh most of the responses are making me feel really inadequate. 7 weeks post partum I was still bleeding, struggling to establish breast feeding and didn't know what more than 30min sleep was. I would not be able to cope with the logistics of moving stuff around to have a house guest, or indeed, having a house guest. And, controversially (this isn't relevant to your case but I keep reading "what about your mum") I think it would be understandable for a woman to be comfortable with her own mum around while she has boobs hanging out and is trying to avoid a blood stain when she stands up but not her MIL. hides for cover
Your initial post made it sound like you had a good relationship with your MIL, and she has done you many favours and essentially, that you owe her. This is definitely influencing responses. From what you said, I have no idea if she is an utter nightmare (belittles you, undermines your parenting, rearranges/takes over everything because, well, it was done her better way etc. whatever nightmare MIL do ), who forces herself round every 6 weeks, kicking you out to have alone time with you kid or is a kindly person who does you a much needed and appreciated favour.
One response I haven't read - is if you DH wants his MIL to stay he could should be taking ownership of the logistics. Not you. Even if he doesn't want her to stay why isn't he sorting this out?
I don't know the size and free space in your house, so am not going to comment on whether she could or couldn't stay. Again MNetters often think in terms of their own homes/experiences, but not all 3 bed houses are equal. Some can be tiny, so fitting changing station in your room may or may not be practical. I have every sympathy for not wanting to change on your bed ,as someone suggested you do. Maybe I was just a bad mum, but I would have ended up with poo and wee everywhere and been changing my own sheets multiple times a day or sleeping in poo and wee. If you have space in the bathroom that may be more practical.
For me, I think 7 weeks is still early days, and the world is still settling. I am very sympathetic to not wanting house guests, even if they are family, at this time. However, you do need, at some point, to work out what the ongoing situation will be. I don't think there is an easy solution (unless a new sofa bed that matches your living room solves all). I can understand MIL not wanting hotels for every visit, but also get that as the little ones get settled in their rooms moving them around will not be ideal (although may have to be done). Is there anyway you can ad hoc partition off part of your open plan living room ? Otherwise move the two little ones into the bigger bedroom and have a single in the box room? I do digress though, as you acknowledged this needed to be addressed and it is not the point you were asking advice on.
Good luck! And don't less this stop you from enjoying this precious time with your little one (congratulations on the birth).