OMG poor lamb I would hate to be you.
Puts me in mind of some years ago - very early 00's - my husband's dad died, and his mum was on her own as his brother had moved to Oz in the late 1990's.
We had one child aged 3, and were in a 3 bed house that we had moved into some months before (from a 2 bed.) We moved as we were running out of room in our tiny 2 bed starter home.
We had quite an argument at the time, as she asked him if she could move in with us! She said she would be 'freeing a 3 bed house up for a family' (her house was council rented.) We had a spare room, and she said she could help by cooking and cleaning, and if we have a 2nd child, the 2 can share for at least 10 years,' 
DH actually wanted her to move in too, saying she would be a good babysitter, she could help with housework and shopping, and would help financially. I got on OK with her, and there was technically room for her, but I just flat out did not want her living with us. She's nice and all, but it would have resulted in a TOTAL lost of privacy, and I would not have been able to relax in my own hone. I would have felt as if I had to entertain her all the time, and chat to her, and take her anywhere she wanted to go as she couldn't drive.... 
I said 'would you want MY mother living here?' My mum and DH didn't like each other much as they were both hot headed and opinionated and sometimes clashed... so DH said 'you KNOW me and your mum don't get on too well, so obviously not - you get ON with my mum!'
He says 'give me ONE reason why my mum can't live here.' It was easy.....
'I don't WANT her living here.......'
I don't know what he said to her, but she stayed put anyway, and we still got on OK, so he must have said it won't work or we are planning another child or something. A few months after all this, she met another man, and he moved in within a few months with his 15 year old daughter. They are still together!
Yep, sorry OP I do not know what to suggest at ALL, as you cannot stop her buying the house.