This is absolutely outrageous OP! Remind your mother that she gave you the money as a gift, and this being the case you spent it on your house. If as others have indicated, she had to sign something when you bought, to confirm that the money was a gift, then remind her of this, and then if she says 'but I need the money now', just say, 'well I'm sorry Mum, but I simply don't have it to give, if you were going to ask for it back you should have said it was a loan, in which case I wouldn't have accepted it, but it's gone toward my house now, so I just DON'T HAVE IT!'
If she tells you to sell your house, then just tell her not to be so ridiculous. If she denies that she's being ridiculous, and insists that you sell your home to give her this money, just say 'Mum, I am NOT going to sell my home, just so that you can live somewhere more expensive! You should never have given us the money if you were going to ask for it back, but now it's gone, and there is nothing to be done about it.' Then get up and walk away.
Sorry OP, but I think you'd be mad to give into this, Mother or not, she doesn't have the right to insist you sell your home, just so that she can live in a posher area.